22 



OUTLINE OF THE ARACHNID THEORY 



The Mesoderm. 



Origin of the mesoderm. To understand the peculiarities of the cephalic 

 mesoderm, we must consider its origin as a whole. 



In Limulus, the mesoderm arises in part from the telopore, a shallow, terminal 

 depression overlying a confused mass of proliferating nuclei destined to form meso- 

 derm, yolk cells, and endoderm. (Figs. 128 and 140.) 



As the embryo elongates, the depression maintains its terminal position, 

 changing to a longitudinal groove, and finally taking the form of a typical primi- 



FIG. 1 6. Scorpion embryos in mercator projection. 



tive streak. (Figs. 129, 130, 140, t.p.}. From the primitive streak, a sheet of 

 mesoderm extends forward and laterally, finally breaking up into somites and 

 lateral plates. 



In the abdominal, or branchiocephalic and vagal regions, the somites are 

 hollow, contain true coelomic cavities, and are quite distinct from the overlying 

 ectoderm. The corresponding lateral plates are sharply segmented, and they 

 are united, for a short period, with the overlying ectoderm. They appear to be 

 formed from the ectoderm by a local, inward proliferation, that takes place, not 

 only in the region of the germ wall, but along the lines that mark the anterior 

 and posterior boundaries of the lateral plates. (Fig. 128, a.) 



On the peripheral margins of the expanding mesodermic area, no segmenta- 



