Procephalic Lobes of Epeira Cinerea. 42Y 



mation of a cone-shaped mass that extends beneath the blastopore 

 downward into the yolk. Text-figure 1, A. The continued forma- 

 tion and growth of cells in this region soon causes the blastopore 

 to disappear, as was pointed out by Morin ; the ultimate result being 

 a pressure from underneath which causes the blastodisc to bulge 

 outward. 



The blastodisc increases in extent by the continued division of its 

 cellular elements in radial planes. In the caudal thickening, how- 

 ever, many of the cells divide in tangential planes, and the new 

 cells derived from this division are pushed forward under the blasto- 

 disc as well as downward into the yolk. Text-figure 1, B. 



The caudal thickening, at this stage, has been likened to a comet, 

 owing to the fact that the lengthening of the blastodisc, or, as it has 

 been designated by Balfour and others, the ''ventral plate," gives it 

 the appearance of passing backward over the surface of the yolk, 

 leaving a constantly widening trail of nuclei in its path. Text- 

 figure 1, C. 



In surface views the nuclei are seen to be arranged in the form 

 of a triangular area covering the ventral surface of the egg. Fig. 6, 

 g. hd. The base of the triangle is a broad plate from which, in a 

 later stage, the cerebral lobes are formed. This structure is the 

 cephalic plate. Figs. 6, 8, and 9, c. pi. 



Sections through the early blastodisc stages show that, from the 

 beginning, there is a rapid increase in the number of cells in the 

 region just beneath the blastopore. This proliferation results, as 

 has been stated before, in the formation of an undifferentiated mass 

 of cells which projects downward into the yolk, and also in the 

 formation of the projecting caudal thickening. From the caudal 

 thickening new cells are formed which spread out as a broad sheet 

 under the blastodisc (w^hich may now be regarded as the ectoderm) 

 thus forming the mesodermal layer. 



Figs. 11, 12, 15, and 10 represent longitudinal sections made 

 . through the blastodisc at this stage. In these figures the cells of the 

 mesoderm (msd.), are seen to arise from the posterior region of the 

 ventral plate, from which point they shift forward underneath the 

 ectoderm. These cells continue to increase in number by division 



