14 



takes its position in the middle of the parent egg. Very soon this nu- 

 cleus separates into two parts which retreat from each other a little 

 way, and soon the rest of the egg, the protoplasm, also divides into 

 two parts, each part forming around one of the new nuclei so that in- 

 stead of one cell there are now two. They are first globular, then oval 

 in shape, and one is a trifle larger, brighter, and harder, than the other. 

 These two cells now divide again, each one making two. These all 

 again divide, and this process is kept up, each cell dividing into two. 

 This process is called the cleavage of the egg. But the cells are of two 

 kinds, and unequal in number. Those derived from the harder and 

 brighter of the first two, partake of the nature of it, and as the division 

 of those cells goes on with more activity than in the case of the smaller 

 and darker ones, when the total number 06 is reached, 64 of them are 

 of the first kind and 32 of the second. The whole mass is in the shape 

 of a ball, the 32 dark cells being in the middle and the 64 brighter 

 ones forming the cover of .the ball. At this stage, the embryo is called 

 a hood gastrula (or amphigastrula). (The name gastrula means "little 

 stomach, " or "little gastrsea. " Gastrsea being a name applied to the 

 primitive intestinal animals, or first animals having stomachs. "Hood" 

 alludes to the shape. "Amphigastrula" also alludes to the shape, 

 "amphi" meaning enveloping, or surrounding.) 



FIG. 20. FIG. 21. 



FIG. 20. Germ vesicle of mammal (rabbit) in tbe stage succeeding gastrulation. a. 

 External egg membrane, cborion. b. Skin layer (oxoderm). c. Heap of dark cells 

 forming the entoderm. 



FIG. 21. A section of tbe above, d. Hollow space within the germ vesicle. (Haeckel.) 



This name is given to distinguish this form of the development of the 

 egg from three other forms which prevail amongst different types of 

 animals. 



The process of cleavage and the formation (in the mammals) of the 

 gastrula, as above, takes place, it is said, in the oviduct of the female 

 on the way from the ovary glands to the uterus. The parent egg, now 

 gastrula, moves into the uterus, and at about the same time the process 

 of segmentation continues to go on till it becomes an expanded globe, 

 the outer cells in a single layer forming the shell as before, and the 



