34 
AN EARLY HUMAN OVUM 
no doubt that the plate of embryonic ectoderm is inturned, and strong 
probability that the condition is a primary one and not due to a precocious 
formation of amnion folds. 
The views enunciated above find their full justification in this the 
earliest phase yet known of the human blastocyst, and lead at once to 
the identification of the larger vesicle as the amnio-embryonic and the 
smaller as the entodermic sac . 1 
It is right however to say that a rather different interpretation 
might possibly be put upon the facts, were the hypothetical stage imagined 
by Minot 2 taken as the initial phase. He represents the amnio-embryonic 
cavity as a transverse slit in a thickened portion of the blastocyst wall, 
consisting apparently of the formative-cell-mass merged with the tropho- 
blast. Our ovum might represent a phase in which the entoderm had been 
separated off, but the amnio-embryonic slit had not yet appeared, and the 
embryonic ectoderm had not been differentiated. The larger sac would in 
this event be the entodermic sac, and the smaller might be considered a 
mere accidental grouping of the cells of the mesoblast. There is no doubt 
however that the smaller sac is closed, and that it is a definite formation. 
The second interpretation is further excluded by the absence in any part 
of the blastocyst wall of any such thickening as represented in Minot’s 
x The views adopted in the text are given at greater length in my article in Quain’s Anatomy , 
lltli ed. 1908, vol. I. Embryology. They are founded mainly on the works of Graf v. Spee, Van 
Beneden, Ilubrecht and Selenka. The chief difficulty which lies in the way of such an inter¬ 
pretation is the presence of an amnion duct in some lower Primates and in Beneke’s ovum and 
the existence of a passage connecting the amnion with the intervillous space in pathological cases 
such as described by Mall. Such a condition may well be secondary, and it is so interpreted 
by Selenka and Keibel, but the appearances might indicate a very precocious formation and 
closure of amnion folds. In my article in Quain, having never seen any distinct recent statement 
on Professor Keibel’s part regarding the amnion, I was led into the error of implying that he 
still held the latter view. In a personal conversation with him however, I learn that he has 
long accepted the primary closure of the amnion in man, and in a proof of his forthcoming Nor- 
mentafel for man, which he has been good enough to send to me, I am glad to find that his 
views are essentially similar to those I have myself been led to adopt. Our case confirms his 
early position (1890) regarding the inversion (reversal) of the layers in the human embryo; there 
is no such reversal in the strict sense of the term, but in so far as the plate of embryonic 
ectoderm is inturned, there is a parallel between the early stages in mice, rats, etc., and those 
in man, as first pointed out, I believe, by Graf v. Spee in 1889. It is now known however 
that this early inturning is a feature of pretty general occurrence in mammals ; true inversion 
(Keimbldtterumkehr) is brought about by secondary conditions which are not present in the case 
of the human ovum. T. H. Bryce. 
' l A Laboratory Text-Book of Embryology , 1903. 
