TIII: PBtt M OF n.K.\\ 



variations are possible here, as well M transitions to c,|iial and un- 

 equal cl.avap'. When the course pursued ix quite typical, the 



Begmentation-nudeuSfBorronnddd by a mantle of protoplasm, lies in 



the middle of the OL r u r in tho nutritive yolk; here it is divided 

 two daughter-nuclei, without 1 ho occurrence of a correspond in-:' In 

 <f the egg-cell. The daughter-nuclei, in turn, undergo division into 

 4. these into S, 1<>, 'M nuclei, etc., while the egg as a wl.olr .-till 

 remains unsegmented. Subsequently the nuclei move apart, the 

 LMvater number gradually migrate to the surface, and penetrate into 

 the protoplasmic cortical layer, where they arrange themselv- 

 uniform distances from each other. It is only at this stage 

 tin- process of egg-segtnentation takes place, for now the cortical 

 v.v divided into as many cells as there are nuclei in it, while the centi-nl 

 yolk /- imiiit* " ' . The latter is therefore suddenly enclosed in 



formed of sin ill cells a blastoderm (Keimhaut). Instead of 

 a polar (telolecithal) yolk, we have a central (centrolecithal > yolk. 

 < )i-(lin u-ily yolk-nuclei or merocytes remain behind in the yolk, as in 

 Ihe meroblastic eggs of Vertebrates. 



No\v that we have become acquainted with the various forms of the 

 process of segmentation, it will be expedient to dwell for a moment 

 on its results. According as the process of cleavage takes place 

 by one or the other of the four methods described, there a 

 a mass of cells with corresponding characteristic*. l-'ivm oipial 

 segmentation there arises a spherical germ with cells approximately 

 uniform in size (Amphioxus, Mammals) (fig. 30, p. 56); from un- 

 equal segmentation, as well as from discoidal, there is produced a 

 form of the geim with polar differentiation. This mai --If in 



the first case (Cyclostomes, Amphibia) in the production of small 

 eells at the animal pole and large yolk-laden elements at the opposite, 

 vegetative pole (fig. 32 64 , p. 60). In the other case (fig. 35, p. 64) 

 the vegetative pole is occupied by an unsegmentrd yolk-ma>>. in 

 which at definite regions nuclei are found ( Fishe-. llrptil.-s, and 

 Birds). Finally there is developed from superlirlal cleavage a germ 

 composed of a mantle of cells, which envelops an unsegmented yolk 

 - in which also there are nuclei (Arthropo 



The mnlticellular gorm undergoes further changes, sometimes in 

 the earlier stages of the cleavau'* 1 process, sometimes only in the later 

 stages, in that a small, fluid-tilled c r \ elopnl in its 



centre, by the -eparation of the embryonic cells. At lir>t .-mall, this 



