Embryonic Development in Man. 375 



layers in early human embryos. The ''Blattumkehr" in monkeys 

 has been defined by Selenka as follows (Biol. Centralbl., Vol. 2, p. 

 552):— 



''Der Embryonalbezirk . . . ist gezwungen sich . . . im 

 Innern der Keimblasse einzustiilpen, wobei das Entoderm zur kap- 

 penartigen Hiille ausgeweitet wird, die Keimbliitter sind daher an 

 dieser Stelle umgelagert, iimgekehrt, invertirt." 



It is now known that no true inversion of the layers occurs in man, 

 but, as stated by Bryce and Teacher, ''there is no doubt that the 

 plate of embryonic ectoderm is inturned, and there is strong prob- 

 ability that the condition is a primary one and not due to a 

 precocious formation of amnion folds." (Page 34.) The primary 

 infolding of the embryonic shield is strikingly shown in Figs. 

 2 to 8. 



Where the embryonic shield forms the floor of the amniotic cavity 

 in sections 156 to 153 (Figs. 9 to 12), there is an opening through 

 the ectoderm in the median sagittal plane of the shield. No other 

 indication of a neurotic canal was found. The opening, however, 

 is presumably an artefact. Similar ruptures through ectoderm are 

 seen in sections 13 and 15. Around the median aperture the ecto- 

 derm is not continuous with the entoderm, as would occur if the 

 structure were a true neurenteric canal. 



The cells of the three germ layers as seen in this embryo may be 

 described as follows : The ectoderm of the shield is composed of 

 more or less cuboidal or cylindrical cells, which are quite epithelial 

 in character. The nuclei are round or oval, with a finely granular 

 chromatin network, and with generally one, occasionally two, distinct, 

 deeply-stained nucleoli. The cell j)rotoplasm is generally very finely 

 granular and stains moderately deeply with eosin. The chromatin 

 network can be particularly well seen in the first sections of the 

 ectoderm shield where the cells have been cut very favorably for 

 this observation and where they are not so densely crowded as 

 elsewhere. The karyokinetic figures are generally in the monaster 

 stage. JSTo distinct diasters were found. One cell was seen with 

 two small vesicular nuclei containing densely but finely granular 

 chromatin, and one pair of cells with the same kind of nuclei and 

 an incomplete division of the protoplasm. 



