No. I.] THE EMBRYOLOGY OF LIMULUS. 49 



Sections of the embryos shown in Figs. 21-23 show but 

 slight differences from those of Fig. 24, and hence a descrip- 

 tion of that will suffice. Fig. 37 represents a section through 

 the anterior end of the streak at stage C. In the median line 

 is the streak itself, which shows a median proliferation of cells 

 extending some distance into the yolk, while on either side is a 

 less conspicuous thickening of the blastoderm. All three of 

 these elements enter into the formation of the appearance of a 

 primitive streak as viewed from the surface, and all contribute to 

 the formation of the middle germ layer. At the point of the 

 section the median ridge extends below the others, and the 

 nuclei at its inner extremity show a tendency to spread towards 

 right and left. Farther back (Fig. 38) these same centres of 

 proliferation may be traced, and here the lateral as well as the 

 median band contributes to the mesoderm. From the primitive 

 streak the mesoderm here extends right and left to the margin 

 of the germinal area, where, apparently, it again connects with 

 the ectoderm. In some sections, especially in later stages, other 

 points of connection occur between ecto- and meso-derm, but I 

 have not been able to trace any regularity in these. 



This account accords well with that of Patten ('90), except 

 that I have failed to trace, in surface view, the ring of mesoderm 

 extending completely around the embryo to which he refers 

 (P- 375)- Probably this is represented by the marginal con- 

 nection between ectoderm and mesoderm in my figure. 



In this method of mesoderm formation a portion of the periph- 

 eral part of the yolk is cut off by the outgrowing middle layer, 

 and comes to lie between it and the ectoderm (Fig. 39). This 

 yolk is in such position that it can readily serve as food for the 

 growing ectoderm, and although I have no evidence on this point, 

 I believe that such is its fate. 



The subsequent history of the mesoderm and its derivatives 

 will be followed in detail in the next portion of these studies. 



Development of External Form. 



Stage D. — The next step is the formation of the appendages. 

 So far as my observations go this process would seem to take 

 place nearly simultaneously on all of the cephalothoracic post- 

 oral segments in the majority of eggs. Yet this is not the case 



