No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 'j T, 



Part II. — The Segmentation and Formation of the 

 Germ-Layers in the Isopods. 



I. The Segmoitation and For7nation of the Genn-Ldyers in 



Jaera. 



Shortly after the extrusion of the second polar globule the 

 first segmentation occurs, its plane passing in the usual man- 

 ner through the point at which the polar globules were ex- 

 truded, and lying, therefore, at right angles to the longer axis 

 of the egg. The division affects, however, only the nucleus 

 and the protoplasm which immediately surrounds it, there 

 being no indication of the division upon the surface of the 

 ovum, a state of affairs which persists through several divi- 

 sions. For the sake of convenience in description, however, 

 the two nuclei with their surrounding protoplasm (PI. V, 

 Fig. 5, A, C) will be spoken of as cells, it being understood 

 that the ovum at this stage and throughout several subsequent 

 stages is in reality a syncytium. 



The second division results in the formation of four cells 

 and is likewise confined to the nuclei and the protoplasm in 

 their immediate vicinity. The axes of both spindles are evi- 

 dently at right angles to the direction which the spindle of the 

 first division held, but after the division is completed the lines 

 which may be imagined as joining the four cells into pairs are 

 not in any case observed by me parallel to each other. Thus, 

 in some cases when two of the cells, derived from the same 

 parent cell, were in clear focus, the other two were also visible, 

 one being in almost equally clear focus while the other was 

 somewhat obscure, and the lines joining the members of each 

 pair were evidently inclined to one another at an angle which 

 varied considerably in different cases. In other eggs, how- 

 ever, when two of the cells were in clear focus only one other 

 could be perceived, the fourth being directly below it and only 

 brought into view by rolling the egg through an angle of about 

 90°; in other words in these cases the lines joining the cells 

 in pairs are exactly at right angles with one another. These 

 two conditions are shown in Figs. 6 and 7 of PL V, the two 



