No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 1 29 



and consequently the same relations obtain between the trocho- 

 phoral and meta-trochophoral regions of Polygordiiis as have 

 been noted as obtaining between the naupliar and meta-naupliar 

 regions of the Crustacea as represented by Mysis and the 

 Isopoda. 



Whether these similar relations indicate an homology of the 

 two regions in the two groups or not I am not prepared to 

 state. If the teloblastic growth is homologous in the two 

 groups, that is to say, if it has been derived from a common 

 ancestor, then it follows that the trochophore and the Naitpliiis 

 are homologous, but, on the other hand, there is no evidence 

 that this is the case ; indeed, the marked dissimilarity in the 

 details of the arrangement of the teloblasts points strongly 

 against any such assumption, and suggests that the teloblastic 

 mode of growth has been developed independently in the two 

 groups, and is to be regarded merely as a provision for rapid 

 growth. The fact that even certain of the organs, such as the 

 nerve ganglia, also show teloblastic growth in the Crustacea 

 and Insects (Wheeler, '91), lends support to this view of the 

 question. 



I have indicated that my results as to the formation of the 

 germ-layers agree essentially with those of Bergh on Mysis. 

 In one important particular, however, there is a difference in 

 our interpretations of the phenomena. As I understand Bergh's 

 statements, the entire mass of blastoporic cells becomes con- 

 verted into vitellophags, endoderm, and eight mesoblasts, and 

 the conclusion is that the mesoderm of the naupliar region is 

 produced by the mesodermal teloblasts. This is certainly not 

 the case in the Isopods, in which, as I have stated, a consider- 

 able number of the blastoporic cells give rise to mesoderm cells, 

 independent of the mesodermal vitellophags, and teloblastic 

 growth is found in connection with neither the mesoderm nor 

 the ectoderm in the naupliar region of the body. We may 

 consider the naupliar region to be the embryo proper, and the 

 blastopore being an embryonic structure gives rise to the em- 

 bryonic mesodermal and endodermal tissues, making provision, 

 however, by the formation of the mesodermal teloblasts for the 

 rapid growth of the later larval mesoderm. 



