A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 1 1"> 



It is not the purpose of this paper to enter into any discussion of the relations 

 between ovulation and menstruation, the passage of the ovum along the tube, or 

 other mooted questions which may have a bearing on the age and development of 

 embryos, but a few words might be said regarding what seem to us to be certain 

 aspects of this subject. Of the three cardinal embryonic features so often quoted, 

 viz, age, size, and stage of development, only the last has any great practical impor- 

 tance, and even here there is more or less variation in different parts of the embryonic 

 body. The fact that age, size, and development by no means run parallel has been 

 pointed out very clearly by Mall (1914; cf. also Rabl, 1915), and indeed there is no 

 reason to expect that they would be exactly comparable. It seems to us that with 

 ova of the same age, dating from the time of fertilization, discrepancies in their size 

 and development may, and not without reason, be assigned to different environ- 

 mental factors. Tubal conditions during migration, varying at different times in 

 the menstrual cycle, might play a certain role, resulting in more or less rapid prog- 

 ress along the tube, accelerating or retarding development. Conditions in the 

 uterus at the time of implantation, premenstrual, menstrual, or post menstrual, 

 etc., time of ovulation, or conceivably the actual size or potentiality of the ripe 

 unfertilized ovum and other unknown or unappreciated factors, might bring about 

 the variations so often observed. The relative independence of age, size, and degree 

 of development is most strikingly evident in the case of pathological ova. 



The variation in size at the same developmental stage is especially marked in the 

 case of embryo No. 1 (see page 116). The chorionic vesicle is roughly of the same 

 dimensions as that of the embryo Kl. 13 of Grosser or of the embryo of Eternod 

 (1898), while the blastoderm of No. 1 is, as well as can be determined, about twice 

 as long as in Grosser's case but of about the same stage of development and a half 

 longer than in Eternod's but far less advanced. In other words, the size of the 

 vesicle is fairly commensurate with the assigned age, while the embryo is dispropor- 

 tionately large, both as regards the chorionic sac and the estimated age; it is, in 

 addition, very large, considering the degree of development. The opposite dispro- 

 portion between the sac and contained embryo is quite characteristic of pathological 

 ova, and it may be that we are dealing here with the results of some subtle influence 

 which has stimulated the growth of the embryo proper without, however, having 

 disturbed unduly its orderly development or brought down the balance on the patho- 

 logical side. A retardation in development but not in growth might account for 

 observed conditions. In the face of the extensive literature on the early chapters of 

 human development we can not claim to present a typical embryo of the middle 

 of the third week, but simply a normal specimen of about that age. It is difficult 

 enough to find one's way among the unnumbered variations of adult morphology, 

 but as regards the embryo we have hardly scratched the surface. It would not be 

 surprising if we had before us in this specimen one of those examples of embryonic 

 variation which are so abundantly present later on (tail, pronephros, milk-ridge, 

 fifth aortic arch, etc.). For obvious reasons these individual variations become 

 more plentiful as development proceeds, but at no time need they occasion any 

 surprise and always may they be ranged under the same rubric. 



