ON THE DEVELOPMENT OF ARANEINA. 61 



flattened shape over the greatest part of the blastoderm, there is one 

 spot where the nuclei become conspicuously spherical and mnlti])ly 

 rapidly. The spot may be distinguished by reflected light as a 

 round whitish area (PL XI, fig. 5, j)rim. th). It is often a little 

 depressed at first ; but it soon becomes flat and eventually a little 

 elevated. Sections through this spot show a large accumulation of 

 blastodermic cells about seven cells deep (PL XII, fig. 11). I shall 

 call this thickening the pririiary thickening. 



Shortly after this another thickening appears, close to the pri- 

 mary thickening, on the future median line (PL XI, fig. 6, PI. XII, 

 fig. 12, sec. tJi). This is also slightly elevated above the general 

 surface of the blastoderm (PL XII, fig. 13). I shall call it the 

 secondary thickening. The primary thickening now gradually ex- 

 tends itself in all directions and forms a whitisli disc-like area of the 

 blastoderm, the centre of which is thicker than the periphery (PL 

 XI, fig. 7). This white area is the first trace of the ventral plate. 

 The primary thickening as it spreads out surrounds and pushes away 

 the secondary thickening, so that the latter now lies at the margin of 

 the white area but is further from the centre of the primary thicken- 

 ing than before (PL XI, fig. 7). 



There has been much confusion in regard to the nomenclature 

 of these two thickenings of the blastoderm. The secondary thicken- 

 ing corresponds to the primitive cumulus as described by Claparede,* 

 This appears at least very probable when we compare my fig. 7, 

 PL XI, with figs. 3 and 4, Planche I, of this author, Balfour was 

 of the opinion that the primitive cumulus becomes lost in the caudal 

 thickening. What is called the primitive cumulus by Locy is 

 undoubtedly the primary thickening above described, while his 

 " caudal thickening " is the secondary thickening. Morin admitted 



* Claparede — Recherches sur I'Kvolutiou des Araignées, Naturk. Verhandel. I. 



