Chap, xiv.] HISTORY OF THE Orr.tf. 527 



protoplasm. When the proportion of protoplasm in 

 any part of an ovum becomes extremely small, seg- 

 mentation does not occur in that part." 



The yolk may be either concentrated in the centre 

 of the egg (centro-lecithal ova) as in many Arthro- 

 pods, or at one pole of the egg (telo-lecithal ova), 

 as in the frog or the chick. In the former case the 

 segments may be all equal, or some may be larger than 

 others (unequal segmentation), or the segmentation 

 may affect only the surface of the egg ; in the latter 

 the segmentation may be unequal, as in the frog, 

 where there is not a large amount of yolk ; or partial, 

 as in the fowl, where, owing to the enormous quantity 

 of yolk that is present, segmentation occurs only in 

 the small area to which the protoplasm is confined ; 

 affording, that is, a proof of the validity of the law 

 just quoted. 



The mass of cells resulting from the segmentation 



O o 



of the ovum may remain closely packed together, and 

 resemble a mulberry (moriila stage), or they may 

 separate from one another, and give rise to a central 

 segmentation cavity (Fig. -215; E, sg) (folastula 

 stage). In the next stage, instead of a single we 

 have two layers of cells, and this condition may either 

 be brought about by the simple inpushing of some 

 of the cells (iiivagiiiatioii), or some of the cells 

 may grow over the others, or the cells of the single 

 layer may undergo transverse division (delamiiia- 

 tioii), and the arrangement of the double layer be 

 thus arrived at. 



As we have already learnt, the two-layered stage 

 with a central cavity is known as the Oastrula 

 stage ; the layers are known as the germinal layers 

 (epiblast and Iiypofolast), and the cavity as the 

 arclieiiteron (Fig. 216). The opening into the 

 archenteron is known as the tolastopore, and 

 appears, from svhat is known of the development of 



