24 CELL-DIVISION AND GROWTH II. i 



In Asteroids and Ophiuroids the division is at first tetrahedral, 

 and to be classed, therefore, with those of the following type ; 

 after the second furrow, however, the blastomeres are rearranged, 

 and division thenceforward is radial. 



In Vertebrata segmentation is altered in megalecithal eggs by 

 the amount of yolk present. It becomes meroblastic ; still the 

 radial type is preserved, though the sequence of the furrows is 

 often altered, the third, for instance, being frequently meridional, 

 and the fourth latitudinal. Amongst the Ascidians Pyrozoma 

 has a large-yolked, telolecithal, and radially segmenting egg. 

 In the Placental Mammals the first two divisions may conform 

 to this type; but segmentation soon becomes irregular. The 

 accumulation of yolk in the Arthropod egg has resulted in 

 a totally different type of meroblastic segmentation. The yolk 

 is here uniformly distributed about the central protoplasm. In 

 the latter is placed the segmentation nucleus, and this central 

 mass divides into a number of cells, which subsequently migrate 

 to the surface and form a blastoderm ; the egg is then centro- 

 lecithal (Fig. 1). The stages of the development of this modifica- 

 tion may be seen in the Crustacea. In certain forms those 

 alluded to above (with the exception of the Cirrhipedes) 

 division is holoblastic and radial. In Gammams, Branchipns 

 (Brauer), Peltogaster (Smith) segmentation is at ^first total, but 

 the inner yolk-containing ends of the cells subsequently fuse. In 

 Crangon, Moina, Daphnella, Daphnia, Orchestia segmentation is 

 superficial. In Isopods and in Decapods segmentation is internal. 

 In all cases the result in the end is the same, a peripheral blasto- 

 derm, a central yolk. But the blastoderm is not always, though 

 it is often, formed simultaneously over the whole surface. There 

 are cases in which it appears first on the ventral side, and by 

 what may be described as a still more precocious formation of 

 the blastoderm, segmentation may begin at this, the future 

 ventral, point, as in Mysis and Oniscns. In these cases the egg 

 is telolecithal. 



In the Insects, Arachnids, Myriapods, and Peripatns novae- 

 zealandiae, the segmentation is meroblastic and the egg comes to 

 be centrolecithal. In Peripatns capensu and in some other species 

 it would appear that the yolk has been secondarily lost. 



