66 K. KISHINOUTE. 



ments of the fourth, third, and second appendages (tig. 13). The 

 mesoblast belonging to the fifth and sixth appendage-bearing segments 

 had united in the median line. 



Formation of the Germinal Layers. 



No gastrula month is found in the formation of the o-ermiual 

 layers, but the blastodermic thickening, shown in figs. 3 and 21, 

 similar to i\\e primary thickening'' of the spider's 1:)]astoderm, may be 

 considered as a modified gastrnla mouth, though in addition to the 

 epiblast it gives rise only to the mesoblast, while the similar thickening 

 of the spider gives rise to both the mesoblast and hypoblast besides the 

 epiblast. lliongh T was nnable to stndy the segmentation of the egg 

 of Limulus, I am inclined t(^ believe that after segmentation the egg 

 is a solid sphere consisting of many splierical balls of similar size, each 

 of which contains a nnclens and abundant yolk granules in it. In 

 one spot of the egg's surface these balls are divided again and again 

 and form the l.)]astodermic thickening. Therefore we find many cells 

 in the interior of the egg. 



In the spider all tlie segmentation nuclei come to the surface of 

 the eo-g, fifter segmentation is over, and form the one-cell layered 

 blastoderm enclosing the yolk which is free from nuclei. At one spot 

 of the blastoderm, cells are \evy rapidly multiplied and form the 

 primary thickening. Some cells forming the lower part of tht; 

 thickening wander in again into the yolk and form the hypoblast. 



These two types in the formatic^n of the germinal layers, exem- 

 plified in Limulus and in tlie spider, are often found in closely related 

 o-roups of Arthropods, and neither of them is of rare «occurrence. 

 I think tliat the type of fjimulus is an abbreviation of that (jf the 

 sjüder. In the f :)rmer tlie liypoblast cells, instead oi coming out of the 



7) Kishinouye— Oh the Development of Araneina. This Journal, vol. lY. 



