654 MANUAL OF PHYSIOLOGY. 



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 

 nutrient material. 



CHANGES IN THE OVUM SUBSEQUENT TO IMPREGNATION. 



The first changes in the ovum independent of impregnation 

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

 and the extrusion from it of certain granular bodies which lie 

 between it and the vitelline 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 yelk. 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 

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

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

 vesicle. 



In the hen's egg the cleavage is confined to the cicatricula or 

 blastoderm, and does not include the rest of the yelk. Such an 

 ovum, from the fact that the cleavage of the yelk is only partial, 

 receives the name of meroblastie. The human ovum, which 

 undergoes complete segmentation, is called a holoblastic ovum. 



The cells in the blastodermic vesicle become arranged into 

 three definite layers, which are called respectively, from their 

 position in the blastoderm, the epiblast, the mesoblast, and the 

 hypoblast. 



From these layers are developed the embryo and the mem- 



