No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 8; 



somewhat more numerous in later than in earlier stages, as 

 may be seen by comparing Figs. 26 and 25 (i^V), but their 

 exact origin I have not been able to determine ; it seems pos- 

 sible that they may be ectodermal cells which the teloblasts 

 have pushed before them in their growth over the surface, 

 a view which their increasing number in older stages would 

 justify. I cannot, however, advance any definite proof of this 

 idea. 



As regards the mesoderm the results obtained from the 

 study of Jaera have been by no means satisfactory, the small- 

 ness of the q^^ rendering proper manipulation exceedingly 

 difficult. It is, however, easy to determine that the mesoderm 

 plug no longer exists in the stage represented in Figs. 24 and 

 25, and that mesoderm cells are to be found in this stage 

 beneath the ectoderm of the naupliar region of the embryo. 

 That these two facts stand in intimate relation with one 

 another I have good reason to believe. Sections show that in 

 stages slightly younger than Fig. 24 the cells of the mesoderm 

 plug are becoming separated and are extending forwards into 

 the region covered by the ectoderm plate. This process con- 

 tinues in later stages, and one finds eventually only a few 

 scattered cells in the region originally occupied by the mesen- 

 dodermal plug, while anteriorly the cells have become quite 

 numerous. The greater part of the mesoderm contained in the 

 mesoderm plug goes accordingly to form the naupliar meso- 

 derm, and so far as could be discovered all the naupliar meso- 

 derm m. Jaera is derived from this source. The liver endoderm 

 in stages later than Fig. 19, is not distinguishable and appar- 

 ently accompanies the mesoderm in its distribution, taking up 

 its position at the junction of the naupliar and meta-naupliar 

 regions where later the liver Anlagen appear. Of the origin of 

 the meta-naupliar mesoderm little could be determined. One 

 finds the mesoderm arranged in masses on either side of the 

 middle line in Fig. 26 {Me), and from what is known to occur 

 in other forms it may be supposed that these masses are the 

 result of a teloblastic division such as Patten ('9o) has de- 

 scribed for Cymothoa. I was unable, however, to make out 

 the mesodermal teloblasts in surface views qI Jaera, and though 



