728 



DEVELOPMENT 



FIG. 500. 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; EN, ento- 

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

 mentation cavity; ZP, zona pellu- 

 cida. (Cunningham.) 



disappears. The next step in the process is 

 the appearance in the yolk of two centro- 

 somes in a clear space near the poles of the 

 elongated vesicle, and the formation of a 

 nuclear spindle, with the aster at either end 

 lying near the surface of the yolk. The 

 nucleus now divides into two parts, and that 

 nearer the surface is extruded from the ovum 

 enveloped in a very small amount of proto- 

 plasm. This forms the first polar body. 

 The nucleus again divides by mitosis, one- 

 half of the chromatin is extruded from the 

 ovum, forming a second polar cell; the chro- 

 matin that remains behind constitutes the 

 female pronucleus. The centrosome has dis- 

 appeared and the ovum undergoes no 

 further changes unless fertilized by the 

 sperm. 



Changes Following Impregnation. 

 The process of impregnation of the ovum 

 has been observed most accurately in the 

 lower types. The process is as follows: 

 The head of a single spermatozoon joins 

 with an elevation of the yolk substance, the 

 tail remaining motionless and then disap- 

 pearing. The head enveloped in the proto- 

 plasm 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 centro- 

 some to the ovum. The centrosome now 

 divides and moves to either side the two 

 pronuclei, a segmentation spindle is formed, 

 and the egg undergoes its first segmentation. 



The process of segmentation begins 

 almost immediately in each half of the 

 yolk, and cuts it also in two. The 

 process is repeated until at last by con- 

 tinued cleavages the whole yolk is changed 

 into a mulberry-like mass, still enclosed 

 by the zona pellucida, figure 500. Fertili- 

 zation probably takes place in the Fallopian 



