gS MCMURRTCH. [Vol. XI. 



It is evident that considerable difference of opinion exists 

 as to the mode of segmentation and the development of the 

 germ layers in the Oniscidas. I have not been able to follow 

 out the development of Porcellio and Armadillidiitm quite so. 

 thoroughly as that of Asellus or Jacra, but my observations 

 are sufficient to allow of a close comparison with these forms. 

 The fertilized but unsegmented egg of Porcellio or Armadilli- 

 diiim resembles very closely that of Asellus, though consider- 

 ably larger. It is enclosed in a chorion and a vitelline membrane, 

 and the surface of the yolk is covered by a thin layer of proto- 

 plasm, while in the center is an amoeboid mass of protoplasm 

 surrounding the first segmentation nucleus (PI. VII, Fig. 39). 

 In the next stage (Fig. 40) two such amoeboid cells are to be 

 found in the egg, which, it may be here remarked, is much 

 more irregular in shape than either Asellus or Jacra, orienta- 

 tion being accordingly much more difficult than in these forms. 

 This 2-celled stage is succeeded by one in which there are four 

 cells (Fig. 41), the arrangement being practically the same as that 

 described m.Jaera ; for instance, the line joining two of the cells 

 {A and B) being at right angles or nearly so with that joining 

 the other two {C and D). I was not able to obtain ova which 

 showed the original two cells in the spindle stage, so cannot 

 state how the arrangement is produced, or even that the cells 

 are correctly identified with those of Jaera; they are identical 

 in position, but whether they are also genetically identical I 

 cannot state. One further point in connection with this stage 

 deserves mention. The peripheral protoplasm, which in earlier 

 stages was uniformly distributed over the yolk, at this stage 

 becomes concentrated to a certain extent at one region of the 

 egg {pp), a region which will later become the posterior 'end 

 of the naupliar portion of the embryo, and may be regarded, 

 therefore, as the posterior pole of the ^'gg. This aggregation 

 of the peripheral protoplasm is an exceedingly interesting pro- 

 cess, marking out as it does thus early a polar differentiation 

 of the egg, and giving also a suggestion as to one of the pos- 

 sible explanations for the belief in an epibolic segmentation in 

 Porcellio. In order that there may be no question as to the 

 centrolecithal character of the segmentation, I represent (Fig. 



