NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/1 



Claparede compares the cumulus to the dorsal organ of many 

 Crustacea. 



Balbiani (No. i) describes the primitive cumulus in Tegenaria 

 douiestica, Epcira diadeuia, and Agelena labyrinthica, as originating 

 as a protuberance at the centre of the ventral surface, surrounded 

 by a specialised portion of the blastoderm (p. 57), which I will 

 call the ventral plate. In Tegenaria domestica he finds that it 

 encloses the so-called yolk-nucleus, p. 62. By an unequal 

 growth of the ventral plate the primitive cumulus comes to be 

 placed at the cephalic pole of the ventral plate. The cumulus 

 now becomes less prominent, and in a few cases disappears. In 

 the next stage the central part of the ventral plate becomes 

 very prominent and forms the procephalic lobe, close to the 

 anterior border of which is usually placed the primitive cumulus 

 (p. 67). The space between the cumulus and the procephalic 

 lobe grows larger, so that the latter gradually travels towards 

 the dorsal surface and finally vanishes. Behind the procephalic 

 lobe the first traces of the segments make their appearance, 

 as three transverse bands, before a distinct anal lobe becomes 

 apparent. 



The points which require to be cleared up are, (i) what is 

 the nature of the primitive cumulus ? (2) where is it situated 

 in relation to the embryo ? Before attempting to answer these 

 questions I will shortly describe the development, so far as 

 I have made it out, for the stages during which the cumulus is 

 visible. 



The first change that I find in the embryo (when examined 

 after it has been hardened) 1 is the appearance of a small, whitish 

 spot, which is at first very indistinct. A section through such an 

 ovum (PI. 31, fig. 10) shews that the cells of about one half 

 of the ovum have become more columnar than those of the other 

 half, and that there is a point (pr. r.) near one end of the thick- 

 ened half where the cells are more columnar, and about two 

 layers or so deep. It appears to me probable that this point is 

 the whitish spot visible in the hardened ovum. In a somewhat 

 later stage (PI. 30, fig. i) the whitish spot becomes more con- 



1 I was unfortunately too much engaged, at the time when the eggs were collected, 

 to study them in the fresh condition ; a fact which has added not a little to my 

 difficulties in elucidating the obscure points in the early stages. 



