1 20 MCMURRICH. [Vol. XI. 



earlier divisions which I have found, and the harmony of my 

 observations with those of Patten ('90) on CymotJioa and those of 

 Bergh on My sis indicate that his observations on this point have 

 not been conducted as carefully as might be desirable. As is 

 well known, Patten ('90) was the first to describe the mesoblasts 

 and their arrangement in the Isopods, and my results agree 

 essentially with his, except that I have not been able to discover 

 the syncytial connections which he figures as existing in the 

 younger rows. I am not prepared to say that the connecting 

 strands of protoplasm do not exist, indeed, I see no reason why 

 they should not, but in none of the numerous preparations I 

 have made of CyviotJioa have I been able to discover them. 



The relation of the ectodermal and mesodermal transverse 

 rows to the segments of the meta-naupliar region is of some 

 interest, and I have been able to determine that eacJi roiv of 

 inesode7'7n cells is eqiiivalent to a segment. Thus, in Fig. 62, of 

 Li^ia, it is clear from the formation of the limbs and the rela- 

 tion of the mesoderm masses to these structures that the meso- 

 derm masses are segmental, and it can also be seen that the 

 masses have been produced by the multiplication of a single 

 transverse row of eight mesoderm cells. In fact the most 

 anterior of the segments represented in the figure is the third 

 thoracic, and the last in which the limb rudiment is visible is 

 the seventh thoracic. Following upon this are seven transverse 

 rows of mesodermal teloblasts which correspond to the six 

 abdominal segments and the telson, the last row of mesoderm 

 cells which, it may be noted, may be regarded as the original 

 mesoblasts, giving rise to the mesoderm of the telson, just as 

 in the annelids the mesoblasts in all probability give rise to the 

 mesoderm of the anal segment. 



Does each transverse ectodermal row also represent a seg- 

 ment .-* This is a question more difficult to answer than when 

 the mesodermal rows were concerned, but I believe that the 

 preparation represented in Fig. 64 indicates what the relation 

 is. This preparation is from a younger embryo of a Cy77iotJioa 

 than that represented in Fig. 63, and though it is difficult to 

 be sure whether or not all the mesodermal segments are com- 

 plete, yet I have reason to believe that they are. This point is 



