That is the form of the soul

http://krishna.terapad.com/resources/3490/assets/documents/Time.doc

Krsna says, janma karma ca me divyam yo janati tattvatah tyaktva deham punar janma naiti mam eti kaunteya [Bg. 4.9]

One who knows the transcendental nature of My appearance and activities does not, upon leaving the body, take his birth again in this material world, but attains My eternal abode, O Arjuna.

So we believe. We blindly believe, blindly or knowingly. It is not possible knowingly, but if we accept Krsna’s argument, then we have to believe. Krsna does not say anything without any reason. Nobody says. No authority says like that. So what to speak of Krsna? Krsna is the greatest authority. If we blindly accept His statement, that is also good, and if we apply our reason and argument, that also you can do. Krsna says in the Bhagavad-gita,

dehino ’smin yatha dehe kaumaram yauvanam jara tatha dehantara-praptir dhiras tatra na muhyati [Bg. 2.13]

As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. A sober person is not bewildered by such a change.

He is giving very practical example that because the soul is within this body, therefore bodily changes are taking place. Now, suppose if a child is born dead. That… You can keep it by some method, preservative method, but it will not grow. One can understand very easily. Because the soul is there, therefore the child from the womb of his mother grows gradually. Grows means changing body. Everything, information, is there in the Srimad-Bhagavatam. The man and woman, father and mother, after sex the two secretion is emulsified and it takes a form like a pea. And the small living entity, the measurement is also given. Kesagra-sata-bhagasya satadha kalpitah: [Cc. Madya 19.140] “One ten-thousandth part of the upper portion point of hair.” You know the point of hair. Now divided into ten thousand part and take one part. That is the form of the soul. That little spark takes shelter into that emulsified pea, and because the soul is there within, it develops from the mother’s womb. The child does not develop all of a sudden. Every mother knows that. It grows gradually, little by little, little by little. When it is seven months, then it is further…, it moves. So in this way the… Krsna says that don’t take this body as the living being. Dehinah asmin dehe. Within this body, the soul is there. So everyone can understand.

Now, because the soul is there, therefore the body is changing. Everyone knows that he was a child. I know, you know. I remember my childhood body or my boyhood body, my youthhood body. I am old man. I remember them, that “I was doing as a young man like this. I was doing as a boy like this. I was jumping. Now I cannot jump. Why? The body has changed.” The body has changed. It is very good logic. The same “I” am there. I was a boy. I was jumping. Now I am old man. I have changed my body. I cannot do that. I will have to take the stick. Because the body has changed. So where is the fallacy of logic? It is very clearly… And the authority says… Krsna says, not an ordinary person. He says, “Within this body there is the soul, and as on account of the soul, the body is changing shape from childhood to boyhood, boyhood to youthhood, from youthhood to old man’s body. Similarly, when this body will be useless, he will accept another body.” This simply truth one has to understand first of all before anything spiritual knowledge. If one cannot understand that the spirit soul is different from this body, then he is cat and dog. He is not human being.

yasyatma-buddhih kunape tri-dhatuke sva-dhih kalatradisu bhauma-ijya-dhih yat-tirtha-buddhih salile na karhicij janesv abhijnesu sa eva go-kharah [SB 10.84.13]

One who identifies his self as the inert body composed of mucus, bile and air, who assumes his wife and family are permanently his own, who thinks an earthen image or the land of his birth is worshipable, or who sees a place of pilgrimage as merely the water there, but who never identifies himself with, feels kinship with, worships or even visits those who are wise in spiritual truth — such a person is no better than a cow or an ass.

Srimad-Bhagavatam 6.1.1
by His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Melbourne, May 20, 1975