MUSEUM OF COMPARATIVE ZOOLOGY. 77 



crease in breadth, though not in length, and the thin membrane persists 

 as the outer layer of the porous membrane, which in the ripe egg bears 

 the villi. Upon the first formation of the villi the appearance of a sur- 

 face view of the membrane so closely resembles that of a porous vitelline 

 membrane (zona radiata) with fine close-set pores, that one must follow 

 the whole course of development, and convince himself of the late ap- 

 pearance of the porous layer before he can be certain that the fine points 

 are due to the villi. 



Kolliker believed that the formation of this peculiar " Dotterhaut " 

 (zona radiata) of fishes could be easily understood as one of the so 

 called secondary cell secretions, if there were on the inner side of it 

 another membrane, which latter would then be regarded as the origi- 

 nal cell membrane of the egg. Although he found some evidence of 

 the existence of such a thin structureless membrane in Cobitis, he 

 was unwilling to give much importance to that fact, but inclined to a 

 belief in its existence, rather upon the theoretical ground that it would 

 offer a satisfactory explanation of the radial pores as being the equiva- 

 lents of pores in cuticular structures. 



This assumption (p. 104) that the fine pore-canals are to be explained 

 as resulting from the presence of fine openings in the cell membrane 

 may be satisfactory enough for those cases in which a definite cell 

 membrane is demonstrable previous to the appearance of the cuticular 

 secretion ; but it seems to me superfluous to assume the universal exist- 

 ence of a cell membrane in order to explain the conditions. Where no 

 cell membrane exists, the same phenomenon may take place, and is no 

 more difficult of explanation than in the case where there is a cell 

 membrane with the supposed structure ; for the latter must in its turn 

 be explained as the result of localized activity on the part of the cell 

 protoplasm in secreting its membrane. So, in the end, it comes to one 

 and the same thing, whether we assume the presence of a cell membrane 

 or not : the explanation must rest on the ability of protoplasm to 

 localize its activities ; but further than that we are at present unable 

 to advance. Why or just how protoplasm is able to effect such a histo- 

 logical division of labor is still unexplained. 



The important paper by Gegenbaur ('61) on the structure and devel- 

 opment of the vertebrate ovum adds nothing to our knowledge of the 

 zona radiata in bony fishes, but is valuable for the way in which it 

 illuminates the subject of the vitelline membrane. 



The only articles of much importance during this decade were one by 

 Buchholz and two by Ransom. 



