No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 4I5 
sary, but variations of this kind are frequently encountered in 
the study of embryology. In the diagrammatic outline of von 
Spee’s embryo v. H. (Fig. 14), I have emphasized the variation 
of the thickness of the ectoderm lining the amnion to corre- 
spond with von Spee’s Figs. 7, 8, 9 in his recent publication. 

Fic. 14. Fic. 15. 
Fics. 14 and 15.— Longitudinal Sections of Two Young Human Ova, after Graf Spee. Enlarged 10 
times. Fig. 14, Embryo vy. H.; Fig. 15, Embryo Gle. Just half of the chorion is drawn, 
and the villi are outlined only over a portion of the ovum. #&, Rauber’s layer; a, amniotic 
cavity ; «v, umbilical vesicle ; ez, entoderm; es, mesoderm; a//, allantois; c, chorda; # c, 
neurentic canal; 4, position of heart. 
His longitudinal section, from which my figure is taken, does 
not emphasize this point, which I consider of importance in 
this discussion. 
In these two ova described by von Spee, the coelom is much 
of the same form it was in embryo No. XI, Fig. 8, and there- 
fore needs no special comment. Yet around the head end of 
embryo Gle. there is a marked accumulation of mesoderm into 
which the heart is to grow. In the illustrations of the section 
of this embryo Graf Spee! pictures spaces in the mesoderm 
which he believes to be portions of the body cavity of the 
1 Graf Spee: His’s Archiv, 1889. 
