No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 91 



unfortunate in that it is just after this stage that the differ- 

 entiation of the mesoderm takes place, and I am obliged to 

 depend upon circumstantial evidence in attempting to ascer- 

 tain its origin. Shortly after the 128-celled stage, the concen- 

 tration of the cells towards the ventral surface, to form the 

 blastoderm, begins. By the time this has become well marked 

 the differentiation of the mesoderm has made itself apparent. 

 In Fig. 34 is represented the youngest blastoderm I observed. 

 It forms a somewhat oval disc, whose longer diameter is at 

 right angles to that of the future embryo. In the center is a 

 patch of cells {vi) which stain more deeply than the rest and 

 which I take to correspond to the vitellophag cells of Jaera ; 

 in front of them lies a row of cells {MEn) somewhat smaller 

 than those still farther forwards, and this row I believe to 

 represent the mesoderm, or more probably, as in Jaa-a, they 

 are mesendodermal. In the blastoderm figured the row seems 

 to be composed of six cells, a number which suggests a close 

 comparison with Jaej-a, in which the same number was found 

 when the mesoderm was first differentiated. As to the orig-in 

 of the mesendodermal cells I can only suppose that they belong 

 to the D group, and that the four D cells of Fig. 32 contain 

 all the mesoderm and endoderm of the future embryo. The 

 small size of the mesendoderm cells, and the fact that they 

 and the vitellophag cells combined are just about sufficiently 

 numerous to account for two divisions of the D group, sug- 

 gests that this number of divisions have intervened between 

 Figs. 33 and 34, and judging from the appearance of Fig. 34 

 there can hardly have been a smaller number, though I did not 

 attempt to enumerate the ectoderm cells. This method of 

 determination from circumstantial evidence of this kind is, I 

 am aware, exceedingly unreliable ; but I believe in this case it 

 allows of accurate conclusions, and that from the X cell of 

 Fig. 31 ectoderm cells and from the D cell mes-endoderm and 

 vitellophags result. 



In a slightly later stage (Fig. 35) the vitellophags are still 

 clearly distinguishable, and the mesodermal row [Me) has be- 

 come much longer, now forming a curved line of about four- 

 teen (J) cells surrounding the anterior portion of the endoderm. 



