THE GERM DISC OR PRIMITIVE CUMULUS. 



225 



partly surrounded by a shallow groove, and situated in the center of a flat germ 

 disc. 



Early in the fifth day, the germ disc forms a prominent, mound-like elevation, 

 or cumulus, with the germinal spot now forming a crater-like depression at its 

 summit. (Fig. 124, A'. A".) In sections, the cumulus appears as a thickening of 

 the blastoderm, with scattering cells arising from the whole of its inner surface, 

 and with a cloud of cells migrating from the central depression, or gastrula, 

 into the yolk. 



pt.c 



A 



-■fxt 



B" 



-p.c 



..--ac 



Fig. 124. — Limulus embryos, seen as opaque objects, in surface views and in profile. The figures show the 

 primitive cumulus, the expanding germinal area, and the beginning of the separation into head and trunk. 



3. Formation of Metameres. 



On the sixth and seventh days, the first traces of metamerism appear. Some 

 of the events that take place at this time are difficult to observe. They are best 

 seen by selecting the most conspicuously marked eggs and examining them by 

 reflected light. 



In stained, surface views of the germinal area, large nuclei may be seen on 

 its margin, sometimes arranged in pairs, two nuclei on the right, two on the left, 

 and one or more on the anterior margin. (Fig. 123, A.pb.l.) These nuclei 

 appear to initiate the formation of the germwall and the periblast, or marginal 

 yolk cells. In later stages, the inner and outer cell layers become a little thicker 

 or darker, over the anterior half of the germinal area, and two germinal spots are 

 now visible, an anterior and a posterior one. (Fig. 123, B., a.c. and p.c.) 



When studied as opaque objects, an earlier stage of the germinal area than 

 IS 



