CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



143 



Each of the pyramidal cells divides tangentially into an inner and an 

 outer cell, both of these cells then continuing to divide. The outer 

 cells form at the periphery a regular layer, the blastoderm (Fig. 65 C, 

 bl), while many of the inner cells lose their regular boundaries. A 

 yolk-mass thus arises in which isolated cells lie (C, d and z). The 

 inner cells, which were evidently the richer in yolk, have now fused 

 to form a common mass. The embryo thus shows a condition similar 

 to that of other Arthropoda, e.g., the Araneae, there being a 

 peripheral layer of cells (the blastoderm) and an inner yolk-mass 

 with cells distributed in it. The latter, indeed, arise in a different 

 way in the Pantopoda, as is shown in Fig. 65 B. The formation of 

 the germ-layers could not be more exactly made out in eggs with 

 equal cleavage, but Morgan assumes that the enteron is formed from 

 the inner cells (the entoderm). In these forms also, Morgan early 



a. 



D6. 



C. 



Fin. 65.— Sections through eggs of Tanystylum (A and B) and Phoxichilidium (C) in the final 

 stage of cleavage (A) and in the stage of delamination and blastoderm-formation (B and C) 

 (after Morgan), bl. blastoderm ; d, yolk-mass ; r, the cells which become detached from 

 the peripheral cells (blastoderm) and shift inward. 



noticed a depression of the peripheral cell-layer, which, like the 

 depression already described in Pallene, he regarded as the rudiment 

 of the stomodaeum. This depression is triangular, a fact which has 

 led to its being compared with the triangular stomodaeum of the 

 Araneae. 



In view of the comparatively slight knowledge which we possess of the first 

 ontogenetic processes in the Pantopoda, it would be too presumptuous to try to 

 form further conclusions. It has already been mentioned that a certain agree- 

 ment with the conditions in the Araneae exists. The splitting of the blastoderm 

 into two layers maintained by Morgan recalls the processes in the Pseudo- 

 scorpiones (p. 28) ; but these also are too little understood to allow of further 

 comparison. The commencement of development and the further differentiation 

 at the one pole might be compared to the formation of the germ-layers in the 



