5i8 KNOWER. [Vol. XVI. 



A study of sections of eggs passing through these stages 

 apparently confirms what is learned from surface views. 



In its early stages the embryonic disc is in cross-section a 

 comparatively broad, flat plate of protoplasm formed by the 

 fusion of its cells, the neighboring cells of the blastoderm 

 being connected rather loosely with the edges of this area 

 (PI. XXXI, Fig. 30). In reaching its final restricted size in 

 PI. XXX, Fig. 18, the broad plate of protoplasm, whose bound- 

 aries were well defined in an earlier section, has become much 

 reduced in extent. The section of the completed disc (PL 

 XXXI, Fig. 31) shows the plate contracted to a decidedly 

 shorter diameter. (The two sections (PI. XXXI, Figs. 30 and 

 31) are drawn to the same scale.) 



The manner in which the mesoderm arises (described further 

 on), partly by a crowding of cells below from the embryonic 

 area as it becomes defined, is another argument in support of 

 the view here advanced for the formation of the first rudiment 

 of the embryo. 



The area of the blastoderm, the origin and gradual concen- 

 tration of which we have thus traced, will be henceforth spoken 

 of as the embryonic area or germ-disc. Though it might be 

 so called at an earlier stage, it hardly merits the term before 

 reaching the definiteness of outline shown in PL XXX, Fig. 18. 



The facts here reviewed appear to me to prove that the 

 embryonic disc is not formed directly in the segmentation by 

 cells wandering toward a predetermined point. The evidence 

 indicates also that the disc is not the result of simply active 

 cell multiplication in a restricted area of the blastoderm. The 

 truth seems to be that segmentation results in the establish- 

 ment of a blastoderm of cells scattered over the entire surface 

 of the yolk, and that then, as these cells increase in numbers, 

 a process of concentration draws many of them together to 

 form an area on the ventral surface, which is the first rudiment 

 of the embryo, the germ-disc. This is shown in the entire series 

 of stages figured, and is brought out vividly by a comparison 

 of PL XXX, Figs. 12 and 13 with PL XXX, Fig. 18. In PL 

 XXX, Fig. 12, the embryonic area spreads over the whole of the 

 posterior half of the ventral surface of the yolk. In PL XXX, 



