142 PANT0P0DA. 



The peripheral cells, which were also formerly present at the opposite 

 pole (A and B), disappear. 



At a time when the blastoderm only partly surrounds the egg, a 

 few cells of amoeboid form are seen lying below it (Fig. 64 C). 

 According to Morgan, cells are given off first at the pole of the 

 micromeres, and then at other parts of the periphery. These cells 

 arise by division of the pyramidal blastoderm-cells in a tangential 

 direction, a process which Morgan, comparing it with the result of 

 observations on other Pantopoda, considers to be one of delamination ; 

 a lower cell-layer forms, which is no doubt to be regarded as the 

 entoderm. This view does not appear sufficiently supported by the 

 facts as yet known, and Morgan's observations have made possible 

 another assumption with regard to the formation of the germ-layers. 

 At the pole of the egg that is richer in cells a thickening appears, 

 which has been compared by Morgan to the primitive cumulus of 

 the Araneid egg (p. 42). A depression then appears at this point 

 (Fig. 64 D, e), and from this an active proliferation of cells takes 

 place. Morgan himself regards this as the formation of the meso- 

 derm, and believes that some of the amoeboid cells which grow into 

 the yolk are also of entodermal nature. The two germ-layers are not 

 yet distinct from one another. In any case, the whole process shows 

 great similarity to the formation of the germ-layers in the Araneae. 

 Amoeboid cells are formed Avhich grow into the yolk, and give rise 

 later to the enteron. That some of the cells which originate near 

 the invagination represent the rudiment of the mesoderm cannot be 

 doubted. These cells soon increase greatly in number, and become 

 arranged into two bands, the mesoderm-bands. The invagination 

 which, on account of its relation to the formation of the germ-layers, 

 might be regarded as the blastopore, is held by Morgan to be the 

 stomodaeum. 



The two genera Tanystylum and PlwxichiUdium, possess smaller 

 eggs less richly provided with yolk, and these differ in their develop- 

 ment from the larger eggs just described, inasmuch as they undergo 

 equal cleavage, by means of which the egg breaks up into two, four, 

 eight, and sixteen blastomeres of equal size. In consequence of this, 

 the pyramidal cells of a later stage are also approximately equal in 

 size (Fig. 65 A). The fact that the yolk contained in such an egg is 

 smaller in quantity than in the other egg leads to a difference in the 

 further development. An actual blastoderm is not at first formed, as 

 in Pallene, but forms later by a process of delamination (Fig. 65 B). 

 A cleavage-cavity also seems to arise, as may be seen in Fig. 65 B. 



