CRANGON VDLGARIS. 139 



endoderm, all of the endodermal cells elongating and 

 budding off cells which go to make up the third germinal 

 layer. The earlier of these mesodermal cells of eiido- 

 dermal originally closely resemble their fellows, but later 

 they are much smaller and stain more deeply. These 

 latter are regarded as comparable with the secondary mes- 

 oderm of Reichenbach. It does not appear that either of 

 these two groups of mesoderm fall under the head of the 

 mesenchyme of the Hertwigs. 



Reichenbach claims that in Astacus the mesoderm is 

 differentiated before the actual gastrulation, and he fig- 

 ures mesodermal cells in advance of the blastopore. This 

 reminds one of Grobben's studies on the Entomostraca. 

 In Cetochilus at the thirty-two cell stage there are already 

 differentiated one mesoderm and two endoderm cells, be- 

 sides two more which are partly mesodermic and partly 

 endodermic, but it is to be noticed that, according to Grob- 

 ben's interpretations, the mesodermic cell is behind the en- 

 dodermic ones. In Moina the same differentiation of 

 mesoderm before invagination is noticed, and the meso- 

 derm occupies the same position, with relation to the 

 endoderm. Here, however, the genital cells are differen- 

 tiated from the other mesodermal ones before either are 

 invaginated, exhibiting an instance of precocity only par- 

 alleled in some Hexapods (vide Witlaczil ('84, pp. 571 

 and 671-677). 



Concerning Lucifer there is some question. According 

 to Brooks ('82) the egg has a regular and total segmen- 

 tation followed by a blastopore stage. At the close of 

 segmentation one of the cells is differentiated from the 

 rest by containing a large amount of food yolk. In in- 

 vagination this cell becomes divided, and the two resulting 

 cells do not take a part in forming the endoderm, but are 

 pushed into the segmentation cavity. Brooks is uncertain 



ESSEX INST. BULLETIN, VOL. XVm. 18 



