656 MANUAL OF PHYSIOLOGY. 



of the chick and its membranes, by far the greater part of the 

 egg, both yolk and albumin, being utilized in supplying the nour- 

 ishment during the subsequent stages of development. 



After the egg has been laid, it obtains no help from the out- 

 side world, except the oxygen of the air and the heat of the 

 mother's body ; it is, as it were, fenced in with a protecting mem- 

 brane, garrisoned with the quantity of provisions required, and 

 by the warmth of the hen's body stimulated to growth and ac- 

 tivity. 



The whole of the human ovum, on the other hand, undergoes 

 segmentation and differentiation in the primary formation of the 

 embryo, which subsequently is supplied with the necessary nour- 

 ishment from the maternal circulation. The life and growth of 

 the human embryo, in fact, depends entirely upon supplies from 

 the mother, the ovum not having within itself any store of nutri- 

 ent material. 



CHANGES IN THE OVUM SUBSEQUENT TO IMPREGNATION. 



The first changes in the ovum independent of impregnation 

 consist in the shrinking of the yolk from the vitelline membrane, 

 and the extrusion from it of certain granular bodies which lie 

 between it and the vitelliue membrane, and are called the polar 

 globules. The germinal spot and germinal vesicle also disappear, 

 and are thought, by some observers, to form these polar globules. 

 After the union of the male and female elements, a new nucleus 

 appears in the vitellus which forms what is called the segmenta- 

 tion sphere. This divides at first into two segments, then into 

 four, eight, sixteen, and so on, until a large mass of cells occupies 

 the yolk, To this condition the name of morula is given, from 

 its supposed likeness to a mulberry. Fluid now collects among 

 the cells, and separates some of them from the others, and the 

 cells arrange themselves into an outer layer and an inner layer, 

 consisting of different kinds of cells. The inner cells finally be- 

 come aggregated at one part of the ovum in contact with the 

 outer cells. The ovum now receives the name of the blastodermic 

 vesicle. 



