No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 119 



from behind forwards, one of the median cells is in process of 

 division, and in the fifth segment its division is complete and 

 its fellow is in process of division. In the sixth segment there 

 are four median cells lying all on the same level, an arrange- 

 ment which indicates a certain amount of migration of the 

 daughter cells, since the division is oblique as is shown in the 

 two segments lying behind. In this sixth segment also two 

 rows of lateral mesoderm cells are found on each side, each 

 row consisting of three cells. The only difference seen from 

 what has been described in Li^ia is that the median cells beofin 

 to divide before the lateral ones, while the reverse seems to be 

 the case in Ligia. In the seventh segment the median cells 

 are again in division, each of the four dividing off a cell anteri- 

 orly, the eight cells so formed arranging themselves later 

 somewhat irregularly, as may be seen from the eighth segment, 

 which also shows that there are nine cells in each of the lateral 

 masses, arranged in three rows of three cells each and proba- 

 bly formed in the manner indicated in the eighth segment of 

 the figure of Ligia. 



The regularity of the entire process of growth of the meta- 

 naupliar region of the Isopods is most remarkable, and the more 

 one studies it the greater is the wonder it excites. The regular 

 rows of ectoderm and mesoderm cells are wonderful in them- 

 selves, and when there is added a more or less definite num- 

 ber of rows for all the species, we see that we are dealing with 

 laws of growth which are at present far beyond our powers of 

 explanation. It is true that the number of ectodermal telo- 

 blasts is not always quite constant, though approximately so, 

 but it is exceedingly interesting to find that where, as in Aselliis, 

 they can be traced back to their earliest differentiation, there is 

 a definite number of them, namely eleven. And this definiteness 

 of number is not confined to the Isopods, but is found also in 

 Mysis (Bergh, '93). As regards the mesoblast, however, the num- 

 ber is more constant, eight and eight only occurring in Cymotkoa, 

 Ligia, and Porcellio ; and again we find exactly the same number 

 in Mysis. Nusbaum ('93) in the figure he gives of their arrange- 

 ment in Ligia, as well as in that he gives of Oniscus, shows 

 neither the constancy of number nor the regularity of the 



