INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



95 



The author makes some general remarks on cleavage, which he sums 

 up in the following table : — 



be \ 



O 



Blasto- /' 

 meres all 

 directly 



con- 

 cerned in ' 

 th(^ form- 

 ation of 



the 

 embryo. 



Blastomeres at 

 first of equal size 

 Cleavage regular 



Blastomeres un- 

 equal from the first 

 Cleavage unequal 

 (Haeckel.) 



During cleavage some cells 

 undergo retrograde metamor- 

 phosis and are converted into 

 food-yolk, which accumulates 

 in the blastocele or in the 

 alimentary canal V 



Cleavage metabolic- 



Blastomeres disposed ^ 

 peripherally : blastocele 

 filled with food-yolk 

 ^ Cleavage superficial 

 (Haeckel.) 



Blastomeres confined 

 to one pole of the egg. 

 Food-yolk accumulates 

 in tlie blastocele, or more 

 usually in the alimentary 

 canal 



-^Cleavage discoidal 

 (Haeckel.) 



Differentia- 

 tion of egg-cell 

 into formative 

 yolk and anu- 

 cleate food-yolk 

 takes place first 

 during cleav- 



Differentiation of egg into egg- 

 cell and anucleute foud-yolk takes 

 place before impregnation. Food- 

 yolk contained in the alimentary 

 ^ canal 

 Cleavage trochoidal. 



Selenka prefers to speak of regular rather than primordial (Haeckel) 

 cleavage, because there is no evidence to show that this mode is really- 

 primitive — and indeed it is rare in the lowest Metazoa, the sponges. 

 He adopts the word metabolic to include those cases in which there is an 

 actual destruction of cells to form food material ; and he distinguishes 

 trochoidal from discoidal cleavage, because in the former the entire 

 egg has no longer the form- value of a single cell, but of a cell plus 

 added food material. Strictly speaking, therefore, the cleavage in this 

 case is entire, not partial. 



In the Echinids studied, as in other Echinoderms, there is an 

 accumulation of coagulable nutritive material in what would other- 

 wise be the cleavage cavity ; even after the formation of the first 

 cleavage plane the two blastomeres are separated by an intermediate 

 layer, so that a morula, in Haeckcl's sense — in which the cleavage- 

 masses are in actual contact — cannot strictly be said to exist. This 

 mode of division and of formation of food-yolk forms a transition to 

 the superficial cleavage characteristic of insects. 



2. Origin of Mesoderm. — At the completion of the blastula stage, 

 the cells at one pole — that at which the gastrula invagination will after- 

 wards take place — are seen to be considerably larger than those at the 

 other pole. Along this pole, and corresponding with the axis of the 

 future gastrula, a cleft appears, thus producing the first indication of 

 bilateral symmetry. On either side of the cleft the (future endoderm) 

 cells divide, and the new cells formed passing into the nutritive ma- 

 terial filling the blastocele, form two symmetrical masses, the foundation 

 of the mesoderm. By the rotation of the embryo the amoeboid cells, of 





o 



