66 EMBRYOLOGY. 



It is the same as that which produced unequal cleavage from 

 equal cleavage ; it is the great accumulation of nutritive yolk, 

 the inequality in the distribution of the egg-substances which 

 goes hand in hand with it, and the alteration in the position 

 of the cleavage-nucleus. The process of differentiation, which 

 is still in a stage of transition in the case of the Frog's egg, is 

 carried to an extreme in the case of the Hen's egg. Protoplasmic 

 substance was already abundantly accumulated at the animal pole in 

 the former case, but in the latter it is still more concentrated, and 

 at the same time has become differentiated from the nutritive yolk 

 as a disc enclosing the segmentation-nucleus. The yolk, accumulated 

 to an enormous extent at the opposite pole, is, in consequence of this 

 separation, relatively poor in protoplasmic substance, which only 

 scantily fills the interstices between the large yolk-spheres. 



Inasmuch as the phenomena of motion duiing the process of 

 division emanate from the protoplasm and nucleus, whereas the 

 deutoplasm remains passive, the active, substance in the case of mero- 

 blastic eggs can no longer master the passive substance and cause it to 

 participate in the cleavage. Even in the case of the Frog's egg a 

 preponderance of the animal pole during cleavage is observable ; 

 within its territory the nucleus lies, the radial figures of the proto- 

 plasm appear, and the first and second planes of division begin to 

 arise, whereas they cut through at the vegetative pole last of all ; 

 moreover the process of division during the later stages takes place 

 there with greater rapidity, so that a distinction arises between the 

 smaller animal cells and the larger vegetative ones. In the case of 

 the Hen's egg, the preponderance of the animal pole is still further 

 increased, and the contrast Avith the vegetative pole is most sharply 

 expressed. The cleavage-furrows not only begin there, but they 

 remain restricted to the territory immediately surrounding it. Thus 

 we get on the one hand a disc composed of small animal cells, on the 

 other an immense undivided yolk-mass, which corresponds to the 

 larger vegetative cells of the Frog's egg. The yolk-nuclei enclosed in 

 the periphery of the germ-disc are equivalent to the nuclei of the 

 vegetative cells of the Frog's egg. 



IP- Partial Superficial Cleavage. 



The second sub-type of partial cleavage is prevalent in the phylum 

 of Arthropods, and occurs in centrolecithal eggs, where a central 

 yolk-mass is enclosed in a cortical layer of formative yolk. Manifold 



