EMBRYOLOGY 1 69 



formative history of animals from germ cell to adult. It paints the pro- 

 gressive panorama of change that cells, tissues, organs, and the body as a 

 whole undergo in attaining their final stages. Another division attempts to 

 explain on the basis of experiment the way in which development works. 



Although the most striking changes in the development of man and 

 mammals occur while the young (first called an embryo and later a fetus) 

 is still inside its mother's womb, yet development by no means ceases at 

 birth. Birth is a mere incident in the whole developmental program. The 

 human newborn is utterly dependent for food and care; many years of 

 infancy and childhood must elapse before it becomes self-maintaining in 

 human society. Only at about the age of twenty-five are the last of the 

 progressive changes complete, whereupon an individual becomes truly 

 adult. 



All vertebrate animals are organized upon a common anatomical plan. 

 Similarly, their fundamental mode of development is essentially identical. 

 While the comparative viewpoint is indispensable for gaining a broad 

 understanding of embryology, it has been of special importance in supply- 

 ing missing pages of the human developmental story. The extent of this 

 reliance on related forms will be appreciated when it is stated that the 

 youngest human embryos known are already embedded in the uterus and 

 possess their three primary germ layers. 



A general concept of how man and other animals develop from a single 

 cell by orderly and logical processes should share in the cultural back- 

 ground of every educated mind. From the theoretical side embryology is 

 the key that helps unlock the secrets of heredity, the determination of sex 

 and organic evolution. The body does not just happen to be arranged as 

 it is. Each end-result in structure is preceded by a definite, developmental 

 course of events. Embryology is able to interpret such rudimentary struc- 

 tures, variations, anomalies and 'monstrous' conditions, as well as to throw 

 hght on the origin of certain tumors and other pathological changes in 

 the tissues. Furthermore, obstetrics is basically merely applied embryology. 



In the middle of the seventeenth century, it was generally believed 

 either that fully formed animals exist in miniature in the eg^, needing only 

 the stimulus of the spermatozoon to initiate development, or that similarly 

 preformed bodies, male and female, constitute the spermatozoa and merely 

 enlarge within the ovum. To be consistent this doctrine of prejorination 

 had to admit that all future generations were likewise encased, one inside 

 the sex cells of the other, like so many Chinese boxes. Serious computa- 

 tions were even made as to the probable number of progeny thus present 

 in the ovary of Mother Eve, at the exhaustion of which the human race 

 would end. The modern teaching, known as epigenesis, was proved cor- 

 rect when von Baer discovered the mammalian ovum in 1827 and later 

 demonstrated the three primary germ layers from which all embryos and 

 their constituent parts develop. 



