Procephalic Lobes of Epeira Ciuerea. 425 



add to the thickness of the cellular layer in this region, and which 

 contribute to the formation of the mesoderm. Accordingly it seems 

 proper to regard the structure as a primitive blastopore (Korschelt 

 and Heider). 



3, Formatioit of (lie Frlmary Thickening, or Frlmiiive Cumulus. — 

 Following the formation of the blastodisc, there is a rapid increase 

 in the number of cells in the region about the blastopore. Cells 

 arising by division from the blastodisc spread out underneath that 

 structure and form a deep layer which has not yet become differ- 

 entiated into ectodenn and mesoderm. 



The cells sink to the greatest depth into the yolk just beneath the 

 blastopore, which is the principal point of increase, and from which 

 they spread out in all directions, extending even to the edge of the 

 blastodisc. 



In unstained preparations, when viewed by reflected light, the 

 blastodisc now appears like a white cap covering the animal pole of 

 the egg. In stained preparations it is seen to be a great accumulation 

 of very closely packed cells. Figs. 4 and 5 ; hid. 



There is some confusion among different authors as to how this 

 accumulation of cells shall be designated. Balfour and Locy unite 

 in calling it the "primitive cumulus," a name that was applied by 

 Claparede to a structure which appears later, and which has been 

 identified as the caudal thickening. Kishinouye, to avoid the con- 

 fusion arising from applying the same name to different structures, 

 has called it the "primary thickening," which name seems l:>est 

 adapted to this purpose. 



4. Formation of the Caudal, or Secondary, Thickening. — Shortly 

 after the formation of the primary thickening, a second thickening is 

 formed which appears as an elevation of the blastodisc near the edge 

 of the blastopore. This elevation is caused by a rapid accumulation 

 of cells at tliat point. Fig. 5, e. fJi. It is to this structure that 

 Claparede refers in speaking of the "primitive cumulus." Balfour 

 recognized its significance and called it the "caudal thickening.'" 



The steps in the formation of the caudal thickening may be out- 

 lined as follows. The first result of the rapid multiplication of cells 

 in the region of the blastopore has already been described as the for- 



