CHANGES FOLLOWING IMPREGNATION 



787 



then disappearing. The head enveloped in 

 the protoplasm then sinks into the yolk and 

 becomes a nucleus, from which the yolk 

 substance is arranged in radiating lines. 

 This is the male pronucleus. The middle 

 piece of the sperm is believed to furnish 

 a new centrosome to the ovum, thus re- 

 storing its capacity for cell division. The 

 centrosome now divides and moves to either 

 side the two pronuclei, and a segmenta- 

 tion spindle is formed, which bears all 

 the chromosomes from both pronuclei. 

 The first segmentation occurs, and divides 

 the egg into two cells, hi each of which there 

 is the unreduced chromosomal number. 



The process of segmentation begins 

 almost immediately in each half of the 

 divided egg, and cuts it also in two. The 

 process is repeated until at last by continued 

 cleavages the whole yolk is changed into a 

 mulberry-like mass, still enclosed by the 

 zona pellucida, figure 502. Fertilization 

 probably takes place in the Fallopian tubes, 

 and segmentation of the fertilized ovum 

 occurs on its passage to the uterus. 



The passage of the ovum from the ovary 

 to the uterus occupies probably eight or ten 

 days in the human. 



The peripheral cells, which are formed 

 first, arrange themselves at the surface of the 

 yolk into a membrane, the ectoderm. The 

 deeper cells of the interior pass gradually to- 

 ward the surface, thus increasing the thick- 

 ness of the membrane already formed by a 

 second, or entoderm, layer of cells, while the 

 central part of the yolk, the blastoderm cavitv, 

 remains filled only with a clear fluid. By 

 this means the yolk is shortly converted into 

 a kind of secondary vesicle, the walls of 

 which are composed externally of the origi- 

 nal vitelline membrane, and within by the 

 two newly formed cellular layers, the blasto- 

 derm or germinal membrane, as they are 



FIG. 502. Conversion of the 

 Morula to the Blastula, For- 

 mation of Blastodermic Vesicle and 

 Membrane. A, Appearance of 

 segmentation cavity and attach- 

 ment of inner cell mass to ectoderm 

 at upper pole of ovum; B 1 , exten- 

 sion and flattening of inner cell 

 mass as it occurs in rabbits and 

 some other mammals; B 2 , exten- 

 sion of entoderm as it occurs in 

 insectivora, monkeys, apes, and 

 man; C, completion of bilaminar 

 blastodermic vesicle; BC, blasto- 

 dermic cavity; EC, ectoderm; EE, 

 embryonic ectoderm; EX, ento- 

 derm; 7, inner cell mass; SC, seg- 

 mentation cavity; ZP, zona pellu- 

 cida, (Cunningham.) 



