EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 73 



EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



Descriptions of graded human embryos will introduce the reader to 

 early mammalian development and indicate the divergencies from the 

 chick stages already studied. A somewhat detailed account of a 4.2 mm. 

 human embryo will then link the fifty-hour chick with the pig studies 

 which follow. 



Referring to the blastodermic vesicle of the mammal (Figs. 17 and 18), 

 it is found to consist of an outer layer, which we have called the trophecto- 

 derm, and the inner cell mass (p. 2 7) . The trophectoderm forms the primi- 

 tive ectodermal layer of the chorion in the higheafmammals and probably 

 in man. Prom the inner cell mass are derikej the primary ectoderm, 

 entoderm, and mesoderm. In the earliest known human embryos, de- 

 scribed by Teacher, Bryce, and Peters, the germ layers and amnion are 

 present, indicating that they are formed very early. We can only infer 

 their early origin from what is known of other mammals. The diagrams 

 (Fig. 74 A and B) show two stages, the first hypothetical, seen in median 

 longitudinal section. In the first stage (A) the blastodermic vesicle is 

 surrounded by the trophectoderm layer. The inner cell mass is differ- 

 entiated into a dorsal mass of ectoderm and a ventral mass of entoderm. 

 Mesoderm more or less completely fills the space between entoderm and 

 trophectoderm. It is assumed that as the embryo grows (B) a split 

 occurs in the mass of ectoderm cells, giving rise to the amniotic cavity 

 and dividing these cells into the ectodermal layer of the embryo and into 

 the extra-embryonic ectoderm of the amnion. At the same time a 

 cavity may be assumed to form in the entoderm, giving rise to the primi- 

 tive gut. At about this stage the embryo embeds itself in the uterine 

 mucosa. In the third stage (C), based on Peters' embryo, the extra- 

 embryonic mesoderm has extended between the trophectoderm and the 

 ectoderm of the amnion, and the extra-embryonic ccelom appears. At 

 first, strands of mesoderm, known as the magma reticulare, bridge across 

 the ccelom between the somatic and splanchnic layers of mesoderm 

 (Fig. 76). The amniotic cavity has increased in size, and the embryo 

 is attached to the trophectoderm by the unsplit layer of mesoderm between 

 the ectoderm of the amnion and the trophectoderm of the chorion. The 

 latter shows thickenings which are the anlages of the chorionic villi, sur- 

 rounded by syncytial trophoderm. In the fourth stage (D), based on 

 Graf Spee's embryo, the chorionic villi are longer and branched. The 

 mesoderm now remains unsplit only at the posterior end of the embryo, 

 where it forms the body stalk peculiar to Unguiculates and Primates. It 

 connects the mesoderm of the embryo with the mesoderm of the chorion. 

 Into it there has grown from the gut of the embryo the entodermal diver- 

 ticulum of the allantois. 



