402 On a Mammalian Ovum in an early Stage. [Mar. 23, 



condition I am unable to say ; in the sections, at any rate, it appears to 

 terminate towards the periphery of the thickened area, and to have become 

 curled somewhat away from the hypoblast. 



I am not aware that a similar structure has yet been noticed in the 

 early blastoderm of any animal ; but its importance in this case in bounding 

 the hypoblast is evident. Indeed, if it should be found that the mem- 

 brane in question is of general occurrence in the mammahan germ, and 

 that the first]] appearance of the mesoblastic cells occurs external to it, 

 as from its proximity to the hypoblast there is little reason to doubt, the 

 fact of the existence of such a film between the commencing mesoblast 

 and hypoblast is strongly in favour of the view which derives the former 

 from cells of the epiblast as against that which would assign to it a hypo- 

 blasts origin, 



[Since writing the above I have been enabled, through the medium of 

 the Royal Society Library, to see a recent number of the new ■ Zeit- 

 schrif t f iir Anatomie und Entwickelungsgeschichte,'which contains a paper 

 by Hensen on the early stages of development of the rabbit's ovum. In 

 sections of one blastoderm, of which he gives figures, there is a delicate 

 line at the anterior part of the germinal area lying midway between the 

 epiblast and hypoblast and marked m.jpr. This is referred to in the 

 Description of the Plate as " vielleicht die erste Anlage der membrana 

 prima." Unfortunately, although all the figures appear to be given, the 

 text of the paper is incomplete (the completion is promised in the next 

 Number) ; and the full description of this blastoderm and the account of 

 the so-called "membrana prima" are for the present altogether wanting. 



But from Hensen's figure I have very little doubt that his " membrana 

 prima " is a structure analogous with that described by me in the cat's 

 ovum as " membrana limitans hypoblastica," although the position and 

 relations of the membrane, especially with regard to the hypoblast, appear 

 somewhat differently in our respective drawings. I think the name 

 " membrana prima " is, however, by no means well chosen for the struc- 

 ture in question ; for, leaving out of consideration the primitive chorion, 

 do not the early embryonic cells (the products of segmentation) them- 

 selves compose the first membrane ? as the very name which has been 

 assigned to them collectively since the time of Pander itself directly 

 implies.] 



DESCEIPTION OF PLATE 10. 



Pig. 1. Diagram of a vertical section across the middle of the developing ovum. Mag- 

 nified. 



z.p. Section of tlie zona pellucida or primitive chorion. 



e.v. Vesicle formed by the epiblast. 



h.v. Vesicle formed by hypoblast much smaller than the former. 



g. Opposite the thickened portion of both layers (germinal area). 



