No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. Z^ 



hardly visible in surface view, though more distinct than that 

 of the vitellophags. 



Sections through this stage, or through one very slightly 

 later (PI. VI, Fig. 21), show that during the concentration of the 

 mesoderm towards the ventral surface it becomes several lay- 

 ers thick, forming a thickening of cells projecting into the 

 yolk, all the cells, however, being well separated from this lat- 

 ter material. In front of this mesodermal "plug" lie the cells 

 of the ectodermal plate {cc) arranged in a single layer, and it is 

 further to be noted that the yolk at this stage contains no cells 

 whatever, nor are any to be found in it by a most careful study 

 of serial sections through ova in what may be termed the blas- 

 tula stages (PI. V, Figs. 13-16). All the cells produced by seg- 

 mentation reach the surface in Jaera, and in this respect my 

 observations are in harmony with those of the more recent 

 students of Decapod embryology, such as Weldon ('92) and 

 Herrick ('92), and in opposition to those of Kingsley ('87). In 

 one batch of eggs taken from a single individual, and which 

 were in about the same stage of development as those repre- 

 sented in Figs. 17 and 18, I did find in several cases a single 

 cell almost in the center of the yolk below the mesoderm band. 

 The arrangement of the superficial cells, however, differed dis- 

 tinctly from those of other eggs in the same stage of develop- 

 ment obtained from several individuals, and I believe these ova 

 to have been abnormal. Whether or not they would have 

 proceeded on to complete development I was, of course, on 

 account of the method employed, unable to determine. 



In a preliminary notice of this paper published some time 

 ago (McMurrich, '92) I ascribed the formation of the mesoderm 

 plug to the division of the mesoderm cells parallel to the sur- 

 face of the ovum, i.e., to delamination. Further observation 

 has, however, led me to doubt the perfect accuracy of this 

 statement. Division of the mesodermal cells forming the plug 

 occurs, it is true, but it seems probable that during the concen- 

 tration of the mesoderm towards the ventral surface some of the 

 cells are, so to speak, elbowed out of their original superficial 

 position and forced below the surface, and to this process rather 

 than to a tangential division I ascribe the formation of the plug. 



