374 Maximilian Herzog. 



rings are composed of three to four to five mesoderm cells; the 

 solid round or oval cords contain a larger number of cells. These 

 formations undoubtedly represent the earliest ''anlage" of the yolk 

 sac blood vessels. The chorion mesoderm and the mesoderm where 

 it extends somewhat into the trophoblast do not yet show any traces 

 of blood vessels. 



The cavity of the yolk sac, which first appeared in section 156 

 (Fig. 9) remains small and slit-like through sections 157 and 158. 

 It then gradually increases and attains a transverse lateral diameter 

 of about 1T6 microns in sections 152 and 150. In section 143, 

 where the last of the "Vorhof" mesoderm is seen, the transverse 

 diameter of the yolk sac is 192 microns. From here on it hangs 

 down free for a considerable distance into the exocctlom. It can last 

 be seen in section 122. Its entoderm and mesoderm layers are there 

 very distinct. The yolk sac must have extended somewhat beyond 

 section 122, but from 121 on it has been lost in the sections. It 

 extended through at least thirty-four, and probably through forty 

 sections, hence its greatest sagittal diameter was between 250 and 300 

 microns. 



The cavity of the amnion, which has a diameter of from 100 to 

 160 microns in the first sections, becomes reduced to a canal in section 

 153, having a lateral diameter of 45 microns and a dorso-ventral 

 diameter of 30 microns. This canal terminates in section 149 (Fig. 

 16). In the first sections of the embryo (Figs. 2 to 8) the amniotic 

 cavity is almost circular. Internally, it is bounded below by the 

 thick embryonic shield, which is curved so as to form a deep crescent. 

 Its concavity is toward the chorion. The ends of the crescent are 

 continued as the very thin amniotic epithelium. However, the 

 amnion is not complete in this region because its two lateral wings 

 stop short and do not meet in the median sagittal plane. This seems 

 clearly due to the artificial rupture of the very thin membrane. With 

 equal certainty it may be said that the great concavity of the ectoderm 

 shield is not artificial, but was present ante-mortem. So thick a 

 layer is not liable to distortion, and its cells show no evidence of 

 disturbance. 



Some embryologists have expected to find an inversion of the germ 



