No. 2.] THE EMBRYOLOGY OF LIMULUS. 1 97 



At first the mesoderm forms a continuous sheet across the 

 ^gZ, but one or two cells in thickness, and united to the ecto- 

 derm in the line of the primitive streak (Figs. 42, 43) ; and 

 frequently there is also apparently a marginal connection be- 

 tween the two layers, the line of junction being indicated by 

 a groove 1 (Figs. 42, 44). This line of union is apparently 

 secondary, and I interpret it as a precocious differentiation of 

 dorsal and ventral surfaces. I have seen no evidence of an 

 augmentation of mesoderm by cell proliferation in this region. 



With the appearance of metamerism the connection between 

 mesoderm and ectoderm is lost, except in the stomodasal re- 

 gion (Fig. 43) and in the abdominal portion of the embryo 

 where new cells continue to be added to the mesoderm for 

 some little time. At first, after the separation of the two 

 layers (Fig. 44), the mesoderm extends as an unbroken sheet, 

 one or two cells in thickness, across the germinal area, the 

 ectoderm outside this area losing its columnar character and 

 becoming more flattened. As yet there is no appearance of 

 coelomic cavities. 



So far as my observations show metamerism obtains its first 

 expression in the ectoderm. Thus Fig. 40 represents a longi- 

 tudinal section to one side of the median line (where the coelom 

 first appears) of an embryo, which in surface view showed dif- 

 ferentiated cephalic and caudal areas, separated by a single 

 somite. The boundaries of these regions are recognizable in 

 the ectoderm of the section marked by arrows, while the meso- 

 derm shows no corresponding metamerism. 



CcELOM. — Owing to difficulties of manipulation I have not 

 been able to correlate surface views and sections, and so can- 

 not say exactly how many somites are outlined when the 

 coelom first appears. It is, however, preceded by a splitting 

 of the mesodermal sheet into right and left halves,^ a condition 

 which is maintained {cf. Fig. 48) until after the appearance 

 of the hmbs. By this splitting are produced two mesodermal 

 bands, the inner margins of which are undulating. As shown 



1 This has already been noticed and figured by Patten ('90, p. 375) who suggests 

 a comparison with similar appearances in vertebrates. 



2 Except in oral and caudal regions. 



