CRANGON VULGARIS. 5 



need revision. He gives a transverse section of his em- 

 bryo, the figure closely resembling fig. 31 of the present 

 paper, and refers to the deeper cells as mesoderm. If they 

 arise, as he claims, by budding from the outer layer cells, 

 it is probable that he is dealing with nervous, rather than 

 mesodermal, tissues. It is, however, more probable that 

 in Mysis, as in other forms, these are true mesodermal cells 

 and have attained their present position by a forward growth 

 in the same way as will be presently described for Crangon. 

 The fate of the vitellophags will be mentioned later. 



The observations of F. H. Herrick ('86) upon the de- 

 velopment of Alpheus record a state of affairs, so far as 

 gastrulation and origin of the mesoderm are concerned, 

 which is readily comparable with that of Crangon. 



Dr. A. T. Bruce, in his complete paper ('87) , is more in 

 accord with the Hertwigs, than was apparent in his prelimi- 

 nary communication ('86) referred to in the second part of 

 this paper. In regard to the formation of the entoderm in 

 Thyridopteryx, this is even more so than in the other forms 

 which he studied. He regards, in all forms, the yolk cells 

 as true entoderm, with vitellophagous functions, and be- 

 lieves the functional entoderm to be of later origin. 



Reinhard has recently ('87) restudied Porcellio scaber, 

 and his results are in fair accord with the interpretation of 

 Bobretzky's researches given in the preceding part of this 

 paper. Reinhard concludes that the egg-nucleus divides, 

 and that some of the resulting nuclei, with a portion of the 

 protoplasm, form amoeboid cells, which gradually creep to 

 the surface. The resulting blastoderm, is not at first con- 

 tinuous, but consists of " islands " (c/. Bobretzky, '74, pi. 

 xxi, figs. 3 and 7). The undifferentiated cells, which re- 

 main behind, form the primary entoderm, which soon be- 

 comes differentiated into ento- and mesoderm. 



Cholodovsky ('88) says that, in Blatta germanica, the 



