238 ALICE THING 



That in the same species this membrane varies to a considerable 

 degree in thickness is shown by figures given below where in the 

 turtle's egg its thickness ranges from 1/x in initial stages to 17 fx. 

 The variation of this dimension in the eggs of different groups 

 of vertebrates can be demonstrated by comparing these figures 

 with those given by Prenant, Bouin, and Maillard (Traite 

 d'Histologie, p. 1097) who quote Nagel's measurements, 1.2/x to 

 1.5fj, in the mouse and 2,0^1 to 2.5// in man. The zona pellucida 

 must be distinguished from the yolk or egg membrane, a ver}" 

 thin membrane observed in some vertebrate eggs surrounding 

 the yolk before maturation is completed (Van Beneden, '80, 

 Fischer '05, p. 595). 



In respect to thoroughness and extent of investigation upon 

 the structure of the zona pellucida, mammalian eggs naturally 

 stand first. Less complete and comprehensive study has been 

 given to it in the lower vertebrate groups. 



HISTORICAL SKETCH 



Authors who have given detailed contributions upon the 

 mammalian zona pellucida may be grouped into three classes: 

 first, those who regard this membrane as originating from the 

 egg cytoplasm; second, those who maintain that it is derived 

 from processes of the egg epithelium or from an exoplastic or 

 intercellular substance of the cells of this epithelium ; third, those 

 who frankly state its origin to be uncertain. The first class 

 includes Van Beneden ('80), Kolliker ('98) and Sobotta ('02). 

 In the normal Graafian follicles of the bat (Rhinolophus ferrum 

 equinum, Schreb.), Van Beneden observed egg cells in such 

 close contact that the zona pellucida of one touched the zona 

 pellucida of its neighbor to the exclusion of the epithelial layers. 

 He, therefore, concluded that the zona pellucida developed from 

 the surface of the egg cytoplasm in the absence of the egg epi- 

 thelium. Kolliker stated that egg cells not yet surrounded by 

 epithelium but already arranged in islets or nests were inclosed 

 in a distinct membrane which he designated as the first anlage 

 of the zona pellucida. 



