No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 105 



embryo certain definite dark patches occur, indicating the pres- 

 ence of special masses of lower layer cells, which mark out 

 imperfectly the naupliar somites. Finally it is to be noticed 

 that in front of the embryonic region proper there is a dark 

 patch {DO ?) situated in the mid-line, which recalls the similarly 

 situated patch occurring in Asellus (Fig. 37, DO?) and may 

 represent the dorsal organ. 



As was stated at the beginning of the paper, I have had for 

 study a certain amount of material representing stages in the 

 development of Cyviothoa and Ligia, but I have not considered 

 it necessary to devote space to a special description of the re- 

 sults obtained from it, both on account of the incompleteness 

 of the material and the general similarity which exists, so far 

 as the material went, between the development of these forms 

 and that of the species just described. Cyviothoa has been the 

 subject of a paper by Bullar ('78), and more recently Nusbaum 

 ('33) has published an account of the development of Ligia 

 occanica, unfortunately, however, concealing his results under 

 cover of the Polish language, the majority of embryologists 

 being obliged to rely, for information as to the contents of the 

 paper, partly on the brief abstract of it which appeared in the 

 Biologisches Centralblatt, and partly on the preliminary notice 

 published by its author in the same journal ('9i). 



In the earliest stage of Cymothoa which I possess the devel- 

 opment has reached a stage which corresponds in all essentials 

 to that of Porcellio represented in Fig. 56, and the later stages 

 up to the formation of the embryo likewise resemble what has 

 been described for Armadillidium in all essential particulars. 

 Thus there is the same absence of differentiation of the mes- 

 endoderm, the same early scattering of the mesendodermal ele- 

 ments, and apparently the same absence of a distinct differen- 

 tiation of eleven primary ectodermal teloblasts. In fact the 

 preparations are so similar to those of Porcellio and Avinadilli- 

 diicm that I have not considered it necessary to figure them. 

 The earliest stage I possess is identical in age with that figured 

 by Bullar, and like that author I cannot make any conclusive 

 statements as to the nature of the segmentation. This much, 

 however, is certain, that there occur scattered over the surface 



