No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. I2I 



not, however, essential for our present purpose. What is of 

 immediate concern is the relation of the mesodermal and ecto- 

 dermal transverse rows. The anterior four rows of mesoderm 

 cells are each separated by an interval corresponding to an 

 ectodermal row, and each corresponds to an ectodermal row, 

 while the row immediately in front of the mesoblast corre- 

 sponds practically to the row of ectoderm cells immediately in 

 front of the ectoblasts. Certain of the ectoblasts are, how- 

 ever, in process of division, and it is clear that this division 

 will result in the interpolation of a row of ectoderm cells 

 between the ectoblasts and the row above which the penulti- 

 mate row of mesoderm cells lies, so that eventually the same 

 relations of the mesodermal and ectodermal rows as exist more 

 anteriorly will be brought about. The conclusion is, then, that 

 two rows of ectodermal cells primarily correspond to each seg- 

 ment and that therefore the rhythm of division of the ectoblasts 

 and inesoblasts is not the same, the ectoblasts dividing tzvice as 

 rapidly as the mesoblasts. It may be pointed out that while the 

 figure which supports this conclusion is slightly diagrammatic, 

 yet nevertheless the relations of the ectodermal and mesodermal 

 cells were carefully indicated by means of the camera, and the 

 figure therefore represents these relations accurately. It is 

 hardly necessary to state that the occurrence of a greater num- 

 ber of ectodermal rows between each of the mesoderm rows in 

 older segments is due to the division of the cells composing 

 the rows ; the numerical relation just described is to be seen 

 only in segments which have been but recently formed. 



Not only is there this definiteness in the number of cells 

 concerned in the formation of each segment, but the number 

 of the segments themselves is definite throughout the entire 

 group of the Isopods. As is well known, there are seven 

 thoracic and six abdominal appendages (not counting the 

 telson) in the Isopods, and if to these be added the segment 

 which bears the maxillipeds and the two maxillary segments, 

 we have in the meta-naupliar region of the Isopods sixteen 

 segments. Since what may be regarded as the primary ecto- 

 blasts and mesoblasts are finally located in the telson, it is 

 clear that diwing development the mesoblasts mnst divide ex- 



