no MCMURRICH. [Vol. XI. 



distinct "segmentation planes." In the Isopods there is no 

 very distinct formation of yolk pyramids, though the appear- 

 ance of segmentation planes upon the surface of the yolk in 

 the 32-celled stage for instance of Jacra, is the equivalent of 

 the process. Physiologically, the occurrence of the yolk pyra- 

 mids is exceedingly interesting, but morphologically it has no 

 further significance than is to be found in typical cleavage. I 

 wish, however, to refer here to the appearance presented in 

 section by ova of Jaera in which the yolk cleavage has oc- 

 curred. In Fig. 20 such a section is represented, and from 

 it the cause of the appearance of the yolk cleavage can be 

 clearly seen. Below the large mes-endoderm cells {MEtt) can be 

 seen a distinct line of protoplasm, also indicated beneath the 

 ectoderm cells {Ec). This line corresponds, when it reaches 

 the surface, with the lines of yolk cleavage, and there is no 

 reasonable room for doubt but that it is the presence of this 

 line which produces the appearance of the cleavage. The line, 

 however, is really a section of a membrane which behaves to 

 reagents in the same way as the general protoplasm and in ad- 

 dition is connected with this by branching protoplasmic fila- 

 ments. Furthermore, and this is an important point, processes 

 run off from the membrane centrally into the yolk, undoubt- 

 edly uniting with similar processes from other cells and produc- 

 ing a protoplasmic network throughout a portion of the yolk 

 outside the limits of the membrane. This membrane cannot, 

 I believe, be regarded as a thin membrane of dead material, 

 i.e., the ordinary conception of a cell wall cannot be applied 

 to it ; it is in reality living protoplasm and consequently it is 

 evident that tJie cleavage of the yolk does itot iiiterjnipt the pro- 

 toplasmic continuity. Whether or not this idea applies also to 

 cases such as Astacns, in which there is a formation of typical 

 yolk pyramids, remains to be seen. 



For some time after the cells have reached the surface of the 

 yolk, their stellate outline indicates the continued existence of 

 the syncytium, but later they appear to round off and actual 

 continuity cannot be demonstrated. This change of appear- 

 ance can be seen by comparing Fig. 48 or 49 of Porcellio with 

 Fig. 53, which shows a later stage in the development of the 



