692 



DEVELOPMENT 



centrosomes 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 sur- 

 face 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 protoplasm. 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 

 disappeared 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 segmen- 

 tation. 



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 continued cleav- 

 ages the whole yolk is changed into a mul- 

 berry-like mass, still enclosed by the zona 

 pellucida, figure 500. Fertilization prob- 

 ably takes place in the Fallopian tubes, and 

 segmentation of the fertilized ovum occurs 

 on its passage to the uterus. 



PIG. 500. Conversion of the Mo- 

 rula to the Blastula. Formation of 

 Blastodermic Vesicle and Membrane. 

 A, Appearance of segmentation cavity 

 and attachment of inner cell- mass to 

 ectoderm at upper pole of ovum; B 1 , 

 extension and flattening of inner cell- 

 mass as it oc urs in rabbits and some 

 other mammals; B 2 , extension of en- 

 toderm as it occurs in insectivora, 

 monkeys, apes, and man; C, comple- 

 tion of bilaminar blastodermic vesi- 

 cle; BC, blastodermic cavity; EC, 

 ectoderm; EE, embryonic ectoderm; 

 EN, entoderm; 7, inner cell-mass; 

 SC, segmentation cavity; ZP, zona 

 pellucida. (Cunningham.) 



