NOtES ON THE DEVELOPMENT OF TUE ARANEINA. 85 



bodies is, however, in the main destined to give rise to fresh 

 cells which enter the blastoderm. 



I have not elucidated to my complete satisfaction the next 

 stage or two in the development of the embryo; and have 

 not succeeded in completely reconciling the results of my 

 own observations with those of Claparede and Balbiani. In 

 order to show exactly where my difficulties lie it is necessary 

 briefly to state the results arrived at by the above authors. 



According to Claparede the first differentiation in Pholcus 

 consists in the accumulation of the cells over a small area to 

 form a protuberance, which he calls the primitive cumulus. 

 Owing to its smaller specific gravity the part of the ovum 

 with the cumulus always turns upwards, like the blasto- 

 dermic pole of a fowl's egg. 



After a short time the cumulus elongates itself on one 

 side, and becomes connected by a streak with a white patch, 

 which appears on the surface of the eg^^ about 90° from the 

 cumulus. This patch gradually enlarges, and soon covers 

 the whole surface of the ovum except the region where the 

 cumulus is placed. It becomes the ventral plate or germinal 

 streak of the embryo, its extremity adjoining the cumulus 

 is the anal extremity, audits opposite extremity the cephalic 

 one. The cumulus itself is placed in a depression on the 

 dorsal surface of the ovum. Claparede compares the cumu- 

 lus to the dorsal organ of many Crustacea. 



Balbiani (No. 1) describes the primitive cumulus in 

 Tegenaria domestica, Epeii^a diadema, and Agelena labyrin- 

 thica, 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 domesiica 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 eumulus 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, (1) what 

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



