42 ARACHNIDA. 



described. It is, however, not impossible that those cases in which 

 it is wanting are not primitive, but are specialised, and that it really 

 is of greater importance than its late appearance in Pholcus and 

 its entire absence in Theridium would lead us to believe. This 

 last view is confirmed by the recently-published work of Kishixouye 

 (p. 44). 



The primitive cumulus* arises as a thickening of the blastoderm 

 (Fig. 22 B), and may project from it as a prominence of considerable 

 size (e.g., in Tegenaria and Agalena, Fig. 23, A and B, p. 46). It 

 has been found in most of the Araneae as yet examined. A depression 

 is said to appear in front of it (Salensky, No. 71, Schimkewitsch, 

 No. 72). We are tempted to regard the latter as the blastopore, at 

 the posterior edge of which the ingrowth of cells is specially active, 

 as in the Scorpiones (p. 2). Some of the statements as to the 

 relation of the primitive cumulus to the germdayers in the process 

 of formation (e.g., those of Bruce, No. 54, and Lendl, No. 63) 

 must evidently be understood in this way. 



If we consider the primitive cumulus to lie at the posterior end of the embryo, 

 we find ourselves in the position which was taken up by Balfour (No. 47). 

 Although, since the time of this writer, the ontogeny of the Araneae has been 

 investigated by several zoologists, very little further light has been thrown on 

 this point. According to the above view, the primitive cumulus corresponded 

 more or less to the future caudal end, the depression lying in front of it, and the 

 cephalic lobes again in front of this (Balfour, Schimkewitsch, Lendl) ; 

 according to another view, the caudal end arises at some distance from the 

 primitive cumulus, the cephalic lobes lying nearer it (Balbiani, Locy). In 

 inclining rather to the view that the primitive cumulus corresponds to the 

 posterior end of the embryo, we are actuated chiefly by theoretical considera- 

 tions. The figures given by Morin and Schimkewitsch seem also to support 

 such a conclusion. It is, however, true that there is little convincing evidence 

 for our assumption that the mesoderm arises from the primitive cumulus. There 

 is, indeed, evidence of active proliferation of cells in the primitive cumulus, but 

 in front of it also (in the region of the future germ-band) the blastoderm appears 

 to be multilaminar (Fig. 22 B). It has already been mentioned that Morin 

 entirely denies this significance of the cumulus. According to him, when such 

 a prominence appears, it arises only after the development of the germ-layers. 

 It cannot, however, be denied that Morin himself represents it as of considerable 

 size (Fig. 22 B). It decreases later by giving off isolated mesoderm cells, and, 

 by degrees, shifts dorsally. This displacement is also evident in Claparede'.s 

 figures, if indeed the prominence seen in them actually corresponds to the 

 primitive cumulus (Fig. 25 A and B, p. 48). That the blastopore, or the last 



* [The authors here use this term in the sense in which it was originally 

 applied by Claparede, i.e., they apply it to the thickening which forms a 

 projection from the surface of the blastoderm, which Kishinouye termed the 

 secondary thickening. Kingsley, on the other hand, terms the primary 

 thickening in Limulus the primitive cumulus. — Er>.] 



