CLEAVAGE AND FORMATION OF GERM-LAYERS. 43 



traces of it, should occupy such a position is from previous evidence improbable, 

 unless we may assume that the proliferating area shifted as the posterior end 

 developed, and thus attained a position which is apparently dorsal. Further ■ 

 discussion of this point is unadvisable, as a glance at the figures of Clatakede, 

 Balbiaxi, Salexsky, Balfour, Schimkewitsch, Locy, and Moein shows 

 that they cannot be brought into agreement with one another. It is evident 

 that the difficulty which attends investigation of this point is the cause of our 

 uncertainty with regard to it. Orientation in the almost spherical egg is 

 rendered still more difficult by the appearance of the different parts of the 

 embryo (cephalic lobes and caudal end) simultaneously with the degeneration of 

 the primitive cumulus. On this account one of the more recent investigators of 

 the ontogeny of the Araneae, Kishixouye, was unable satisfactorily to decide 

 the position of the primitive cumulus in the embryo. We must, for the present, 

 accept with some hesitation the view that the depression which appears in the 

 blastoderm of the Araneae and the primitive cumulus corresponds to gastrula- 

 tion, although such an interpretation appears very probable, especially when 

 comparison is made with the Scorpiones. 



This subject is not exhausted with the question as to whether the germ-layers 

 originate in a region corresponding to the later ventral surface, in which the 

 primitive cumulus represents an area of active cell -proliferation (perhaps a 

 point of ingrowth), for there exists a different interpretation of the origin of 

 the germ-layer. According to the view given above, it is to be assumed that 

 the cleavage-cells shift to the periphery to form the blastoderm, and that the 

 germ-layers originate there by an ingrowth of cells (Figs. 21, F, and 22, A and 

 B). While the mesoderm remains as a compact accumulation on the ventral 

 side, the cells of the entoderm become detached from it and shift into the yolk ; 

 from these the enteron forms later. The origin and the fate of these yolk-cells 

 is otherwise described by Balfour, Schimkewitsch, Locy (?). The most 

 important point in these diverging views is the assumption that some of the 

 cleavage-cells remain in the yolk. These cells, which are not utilised in the 

 formation of the blastoderm, do not represent the entoderm alone, but some of 

 them give rise to mesoderm-elements (Balfour, Schimkewitsch). 



According to Schimkewitsch, cleavage and the formation of the blastoderm 

 take place in such a way that the egg breaks up into a large number of yolk- 

 pyramids in the manner already described. Each of these pyramids contains a 

 nucleus which at first lies at the centre. The nuclei shift to the periphery later, 

 and there, with the protoplasm which surrounds them, become separated from 

 the yolk. An outer cell-layer, the blastoderm, is thus formed. It appears, 

 however, as if a further division of the nuclei had taken place previously, and a 

 large number of nuclei had remained within the yolk ; at least, this is what we 

 understand from Schimkewitsch's description of the cleavage-process.* During 

 the development of the blastoderm there is a further increase in the number 

 of the nuclei which remained within the yolk. Before following its further 

 fate, we must mention a process which was observed by Schimkewitsch in 

 Araneid eggs, and had been previously noticed by Salexsky. The blastoderm - 

 cells which at first surround the egg, shift towards the ventral side, and there 



* The statements of Schimkewitsch as to the breaking up of the yolk- 

 pyramids and the formation of uninuclear and multinuclear yolk-cells do not come 

 within our scope, and also require corroboration. As a whole, his figures agree 

 with the descriptions of earlier writers. Schimkeavitsch also found the central 

 cleavage-cavity in a few forms ( Tegenaria, Epeira), and describes it as filled with 

 masses of yolk, in the way described for Theridium (Fig. 21, C and D). 



