SEGMENTATION OP THE OVUM OF THE SHEEP. 235 



quite normal and uninjured if Duval had mentioned whether 

 the zona radiata was present and quite sound. 



Of course Duval notices this all-important difference, but 

 cannot explain it. He says (p. 155), '' Quant au blastopore 

 primitif (ombilic ombilical) du Murin, nous devons aussi 

 remarquer que nos preparations nous le montrent avec des 

 rapports differents de ceux observes par van Beneden. Get 

 auteur Fa decrit et figure comrae place au centre de la region 

 ou la masse endodermique adhere a la face interne de la sphere 

 ectodermique. Nous Favons vu, ou, pour mieux dire, nous avons 

 vu un orifice, precisement dans la region opposee, au centre de 

 I'autre hemisphere. Nous ne saurions pour le moment ex- 

 pliquer cette contradiction. Nous sommes bien convaincu de 

 la valeur et de I'exactitude de la fig. 24 (pi. i), I'oeuf que I'a 

 donnee, etant conserve en coupes, qui ont pu etre etudiees a 

 diverses reprises. Mais nos observations ne sont pas assez 

 nombreuses, car, entre le stade F et la stade H nous n' avons 

 qu'une observation." 



It seems to me that there is a greater weight of evidence in 

 favour of van Beneden's view. 



Taking the combined results of Duval's and van Beneden's 

 papers, the interpretation off'ered in my diagram, V — V^, 

 PI. 18, seems to be by no means an impossible one. W is 

 Duval's fig. 15, 17, 18, or 20, reversed, E V is derived from 

 fig. 21, and the description of the same on p. 140. Duval 

 concludes that his fig. 21, H., is a section taken through such a 

 specimen as his fig. 20, at right angles to the direction of the 

 section fig. 20, close to one end, and that it cuts the ectodermal 

 cells only. Now according to Duval this ectodermal layer is 

 only one cell thick. Is the section Duval has drawn (fig. 

 21, H.) taken as close to the surface as he supposes ? Is it not 

 possible that the ectoderm is really two layers thick (or perhaps 

 more) at this end of the embryo like my diagram E V ? Fig. 

 21, H., undoubtedly shows an inner core of cells, and when we 

 consider how convex a surface it is which is supposed to be 

 cut, it is remarkable that in no less than fifteen out of seventeen 

 cells through which the section passes the nuclei should have 



