862 



CLEAVAGE OF THE YELK. 



[The union of the representatives of the male and female elements forms the first 

 embryonic segmentation sphere or blastosphere, which divides into two cells, and 



these again into four, and so on (fig. 658).] 



In Echinoderms, 0. Hertwig and Fol observed that 

 several embryos were formed when, under abnormal con- 

 ditions, several spermatozoa penetrated an ovum. The 

 male pronuclei, formed from the several spermatozoa, 

 then fused each with a fragment of the female pro- 

 nucleus. Under similar circumstances, Born observed 

 in amphibians abnormal cleavage, but no further de- 

 velopment. 



Cleavage of the Yelk. In an ovum so fer- 

 tilised the yelk contracts somewhat around the 

 newly-formed nucleus, so that it becomes slightly 

 Egg of a Star-fish (Asteracanthion) with separated from the vitelline membrane, and for 

 two extruded polar globules. Male the first t j me t h e nuc leus and the yelk divides 

 and female pronuclei near each other. .^ twQ nucleftted sphere8 . This process is 



spoken of as a complete cleavage or fission (fig. 658). Each of these two cells 

 again divides into two, and the process is repeated, so that 4, 8, 16, 32, and so on, 



Fig. 657. 



Fig. 658. 

 Segmentation of a rabbit's ovum, a, two-celled stage ; b, four-celled stage ; c, eight-celled 

 stage ; dc, many blastomeres showing the more rapid division of the outer-layer cells, and 

 the gradual enclosure of the inner-layer cells ; ect, outer-layer cells ; ent, inner-layer cells ; 

 pgl, polar globules ; zp, zona pellucida. 



spheres are formed (fig. 659). This constitutes the cleavage of the yelk, and the 

 process goes on until the whole yelk is subdivided into numerous small, nucleated 



spheres, the " mulberry mass " or " seg- 

 mentation spheres " or " morula," or the 

 protoplasmic primordial spheres (20 to 25 

 /x) which are devoid of an envelope. [Each 

 cell divides by a process of karyokinesis. 

 According to Van Beneden, the segmenta- 

 tion begins in 1-2 hours after the union 

 of the pronuclei, and the process is com- 

 plete in about 75 hours. These primi- 

 tive cells, from which all the tissues of the future embryo are formed, are called 

 blastomeres.] 



Fig. 659. 



Cleavage of the yelk of the egg of Anchylo- 



stomum duodenale. 



