THE PROCESS OF CLEAVAGE. 67 



variations are possible here, as well as transitions to equal and un- 

 equal cleavage. When the course pursued is quite typical, the 

 segmentation-nucleus, surrounded by a mantle of protoplasm, lies in 

 the middle of the egg in the nutritive yolk ; here it is divided into 

 two daughter-nuclei, without the occurrence of a corresponding division 

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

 4, these into 8, 16, 32 nuclei, etc., while the egg as a whole still 

 remains unsegmented. Subsequently the nuclei move apart, the 

 greater number gradually migrate to the surface, and penetrate into 

 the protoplasmic cortical layer, where they arrange themselves at 

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

 the process of egg-segmentation takes place, for now the cortical layer 

 is divided into as many cells as there are nuclei in it, while the central 

 yolk remains undivided. The latter is therefore suddenly enclosed in 

 a sac formed of small cells a blastoderm (Keimhaut). Instead of 

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

 Ordinarily yolk-nuclei or merocytes remain behind in the yolk, as in 

 the meroblastic eggs of Vertebrates. 



Now 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 arises 

 a mass of cells with corresponding characteristics. From equal 

 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 germ with polar differentiation. This manifests itself in 

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

 cells 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 unsegmented yolk-mass, in 

 which at definite regions nuclei are found (Fishes, Reptiles, and 

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

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

 mass in which also there are nuclei (Arthropods). 



The multicellular germ undergoes further changes, sometimes in 

 the earlier stages of the cleavage-process, sometimes only in the later 

 stages, in that a small, fluid-filled cleavage-cavity is developed in its 

 centre, by the separation of the embryonic cells. At first small, this 



