A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



409 



usually in a better state of preservation than the embryo. There will be considered 

 here only presumably normal embryos in which the somites have not yet made 

 their appearance. These will be taken up in three groups: (1) Those in which the 

 primitive groove is not yet formed ; (2) those in which the primitive groove is pres- 

 ent; and (3) those having, in addition to the primitive groove, a neurenteric canal 

 and medullary folds. A list of these, with their measurements, is given in table 1. 



TABLE 1. Dimensions of human ova of the presomite period. 



* Embryonic mass. 



t Measurement taken from author's illustration. 



GROUP 1. EMBRYOS BEFORE THE FORMATION OF A PRIMITIVE GROOVE. 



The youngest stage in the development of the human embryo that has thus 

 far been observed is represented by a blastocyst already embedded in the uterine 

 mucosa, but devoid as yet of villi. Two such specimens have been described, one 

 by Miller (1913), the other by Bryce and Teacher (1908) . The one described by Miller 

 is smaller and in it the embryonic rudiment consists of a solid mass of cells, whereas 

 in the Bryce-Teacher specimen an amnio-embryonic vesicle can be recognized. 

 Certainly the Miller specimen must be regarded as normal, and although the Bryce- 

 Teacher specimen shows some evidence of degeneration it also should be provision- 

 ally regarded as normal. When more specimens of about the same age are available 

 for comparison it is quite possible that the evidence may prove that the latter is 

 not normal. 



The specimen described by Miller was found in sections of material obtained 

 at curettage. It was not a complete series, only five sections showing the ovum. 

 Fortunately, however, three of these passed through the embryo, which is repre- 



