78 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



B. Partial Division — Meroblastic Segmentation 



(3) Eggs with a large quantity of yolk on which the formative 



protoplasm lies as a small disc at one pole, divide partially, 



and in discoidal fashion, 



[or. Ova which are telolecithal. and have a large quantity of 



deutoplasm. undergo meroblastic and discoidal segmentation]. 



This is illustrated in all Cuttle-fishes, all Elasmobranch and 



Teleostean Fishes, all Reptiles and Birds (Fig. 39 (3)), 



and also in the Monotremes or lowest Mammals. 



(4) Eggs with a considerable quantity of yolk accumulated in a 



central core and surrounded by the formative protoplasm, 

 divide partially, and superficially or peripherally, 

 [or, Ova which are centrolecithal undergo meroblastic and peri- 

 pheral segmentation.] 



This is illustrated by most Arthropods (Fig. 39 (4)), and 

 by them alone. 



Cleavage pattern. — After fertilisation, and before the 

 division of the egg into the two first " blastomeres," there 

 may be a visible rearrangement of the materials in the 

 cytoplasm. The subsequent cleavages very often follow 

 so regular a pattern that it may be possible to point to a 

 particular region of the cytoplasm of the ovum and predict 

 the part which it is to play in the formation of the embryo, 

 if development follows its normal course. But we must 

 be chary of supposing that any such region is specialised 

 from its surroundings except by position, for it is often 

 possible to obtain complete embryos from fragments of 

 eggs, or from isolated blastomeres from the two-cell or 

 four-cell stage. Eggs which have this power of readjust- 

 ing their organisation are called " regulative " in contrast 

 to the " mosaic " eggs in which there is more evidence 

 of the presence of a fixed structural pattern. Even here, 

 however, it is too much to speak of " organ-forming 

 substances " in the egg ; for it is usually found that the 

 visible pattern of the cytoplasm may be completely changed 

 by whirling the eggs in a centrifuge, without marked 

 abnormalities in the subsequent development. 



The plane of the first cleavage is typically a meridian 

 running through the two poles of the egg, and its exact 

 situation is determined by the path of the spermatozoon 

 nucleus and centrosome in the cytoplasm. For subse- 

 quent cleavages, the simplest type is seen in the sea-urchin 



