ON THE DEVELOPMENT OF LIMULUS LOXGISPIXA. 67 



yolk and afterwards returning to it a.s in the spider, remain in it 

 from the time of their formation. 



The epiblast iu Liniulus is derived in t\V(3 diiferent ways. The 

 epiblast of the ventriim arises from the superficial layer of cells of the 

 blastodermic thickening, separating from the lower cells, while the 

 epiblast of the dorsum is difl^'erentiated from the immediately under- 

 Iving hvpoblast cells by rapid nuiltiplication, thus forming a single 

 layer of columnar cells (fig. 41). 



The mesoblast of Limiilus has three sources. One portion arises 

 from the cells forming the lower part of the blastoderm thickening. 

 This portion forms the mesoblast of the céphalothorax. The second 

 portion arises from the primiti"S'e streak. After the separation and 

 difi'erentiation of the cells of the blastodermic thickening into the epiblast 

 and mesoblast, and after the metamerism of the ventral plate has 

 already begun, a median longitudinal groove, the primitive streak, is 

 found near the posterior end of the ^'entral plate. From the bottom 

 of the groove many cells become loose and proliférai e vigorously 

 between the epiblast and hypoblast. These cells form the mesoblast 

 of the abdomen (probably the mesoblast of the last thoracic segment 

 also). The primitive streak is fomul even in an embryo about three 

 weeks old (fig. 32), Tt recedes gr;idually ]_)osteriorly giving rise to 

 paired sheets of the mesoblast. It may be compared to the secondai'v 

 thickening, the caudal thickening, of the spider's l)lastoderm, though 

 the latter is produced before the difi'erentiation of the germinal layers, 

 thus of course before the appearance of metamerism. The primiti^•e 

 streak of Limulus is probably homologous with the structure of the 

 same name in the Chordata. The section shown in fio-. 27 has a strik- 

 ing resemblance to a section across the primitive streak of a chick. 

 The third portion of the mesoblast arises from some yolk cells. In 

 later stages some yolk-cells losing the yolk granules come out of the 



