CRANGON VULGARIS. 6 



dermal extension, and he regards tlie latter as limiting the 

 posterior extension of the potential blastopore, and forming 

 the anal lobe, while the white spot {tacke hlanchdtre)h the 

 cephalic end of the future embr3^o. The mesoderm has a 

 varying origin, according to his text. In some fornis it 

 arises from the blastoderm, mach as described by Morin, 

 while in. others it is produced by bndding from the poly- 

 nuclear yolk pyramids. The plates, however, do not seem 

 conclnsive on the latter point but are apparently capable of 

 being interpreted after the manner of Morin. A recon- 

 ciliation of their difFerent accounts of the origin of the en- 

 toderm is, however, more difficnlt. Schimkewitsch studied 

 Agalena, Lycosa, Pholcus, Epeira and Tegenaria. 



Josef Nusbanm has given('87) a brief account of some 

 of the earlier stages of the development of Mysis chameleo, 

 but if we are to accept his interpretations of his observa- 

 tions, his description of the origin of the germ layers is 

 not easily reconciled with what is known of the coenogeny 

 in any other arthropod. He says that the egg before seg- 

 mentation is surronnded l)y a blastema and has its nucleus 

 at the formative pole. The hrst segmentaition produces 

 two cells, one of which gives rise to the blastoderm, while 

 the other sinks into the yolk. The larger central cells of 

 the blastoderm later bud off other cells which also sink into 

 the yolk and together with the productsof the division of the 

 first cell migrating to that region give rise to " vitellophags " 

 the function of which is the modilication of the yolk. Now 

 first appear the rudiments of the germ, the figure which 

 he gives closely resembling my tig. 10. At this time a 

 shallow invagination(c/'. my fig. 231, a/)takes place in the 

 caudal area, and the invaginated cells, undergoing a rapid 

 proliferation, from a solid entoderm. Then, hehind the 

 point of invagination the abdomen is budded forth. The 

 "vitellophags"(to which we shall return later in the present 



