140 THE DEVELOPMENT OP 



whether the whole of the primitive yolk-bearing cell be- 

 comes thus pushed in or whether only its deeper end is 

 segmented off and placed into the blastoccele. The fur- 

 ther history of these cells was not traced. It would seem 

 probable that they go to form the mesoderm, and, if so, 

 they would afford another instance of early differentiation 

 of that layer. Brooks, however, is inclined to the be- 

 lief that they go to form a food-yolk like that of other 

 decapods. His reasons, however, are far from apparent. 

 In the other groups of Crustacea the information as to 

 the origin of the mesoderm is extremely scanty. In 

 Oniscus (Bobretzky, '74), it would appear that both mes- 

 oderm and endoderm are formed at the same time and are 

 differentiated by a delamination at a later date. Bullar's 

 results in Cymothoa do not differ greatly from this. Nus- 

 baum's account of the process in Oniscus is summarized 

 above. New observations are, however, greatly needed, 

 for these accounts lack greatly in desirable details, and it 

 will possibly prove that the origin of the mesoderm in all 

 these meroblastic types does not differ greatly from that 

 of the more common uninvaginate forms. 



EXTERNAL DEVELOPMENT. 



From this point where the three germ layers are devel- 

 oped, it is necessary to follow out the different organs 

 separately, since the whole cannot be described at once. 

 In order to have a means of correlating the stages of 

 growth of the different systems, I give first an outline of 

 the external features up to the time of hatching, leaving 

 the internal organs until a later time. I may sny at the 

 outset that I do not attempt to trace the various modifica- 

 tions of the appendages in detail, but merely to figure 

 and describe the general external appearance, so that the 



