lOO MCMURRICH. [Vol. XL 



The 32-celled stage of Porcellio I have not seen, but in 

 Armadillidiimi (Fig. 46) the two polar cells have divided so as 

 to form four cells arranged as shown in the figure, and around 

 them is a circle of eight cells whose origin I have not made 

 out. The appearance of the ova at this stage is markedly dif- 

 ferent from that of Jacra, and presumably Asellus also, on 

 account of the total absence of superficial segmentation lines. 

 Indeed, in neither Porcellio nor Ai'inadillidiinn does a segmen- 

 tation of the yolk ever occur, a condition no doubt explained 

 by the greater amount of yolk presented in these forms. It 

 was probably some such stage as this which Roule mistook for 

 an unsegmented ovum. 



The next stage which I have observed corresponds to the 

 64-celled stage of Asellus, though whether there are actually 

 sixty-four cells present I cannot state. The most striking 

 feature of the stage (Fig. 48) is the commencement of a con- 

 centration of a number of the cells around a point which in 

 some cases is on the side of the egg, and in others at its ex- 

 tremity. The concentration is indeed but feebly marked at 

 this stage, but is nevertheless indicated, and becomes pro- 

 nounced in the next stage. In the center of the region around 

 which the concentration is taking place _/i?//r cells (D^-D^) may 

 be observed which differ somewhat from the others in their 

 staining properties. These may be the four polar cells of the 

 preceding stage or else four of the eight cells which would be 

 produced from their division. Which of these alternatives is 

 correct I cannot state, but from analogy with what occurs in 

 Asellus I am inclined to believe the second one the more prob- 

 able. These cells give rise to the mes-endoderm, and in the 

 next stage (Fig. 49) have increased to eight in number, -the 

 concentration of the other cells increasing. These last two 

 figures (Figs. 48 and 49) represent ova of Porcellio; I did not 

 find Armadillidiimi ova in corresponding stages, but the next 

 figure (Fig. 50) represents an egg of that genus in a slightly 

 later stage. The mes-endoderm is formed of a much larger 

 number of cells, and the concentration of the ectodermal cells 

 around it is very evident. In Figs. 51 and 52 slightly later 

 stages of Armadillidiu'm are shown, presenting a still greater 



