A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 411 



In the well-known Peters ovum, which is much more completely described, 

 the conditions are closely similar to those in the ovum just referred to. In it the 

 villi are more numerous and distinct and the chorionic vesicle is larger. In form 

 and position this embryo is almost identical with the Linzenmeier specimen, the 

 amnion lying in contact with the mesoblastic layer of the chorionic membrane. 

 Both possess closure-caps (Gewebspilz) consisting of partially organized fibrin and 

 blood, into which the trophoblast sends branches. 



As will be seen, the embryos of the first group, before the formation of the 

 primitive groove, fall naturally into three substages: (1) those without villi, (2) 

 those possessing primitive villi, and (3) those in which the villi have already begun 

 to branch. The first two have just been considered, and we may now take up those 

 with branched villi. There are three ova, described respectively by Fetzer (1910), 

 Heine-Hofbauer (1911), and Jung (1908), which are a little larger and a little further 

 differentiated than that of Peters. The three are very much alike in form and 

 about the same size. They have numerous and well-defined chorionic villi, which 

 show a beginning tendency to branch. In all of the specimens the yolk-sac is smaller 

 than the amniotic cavity and its wall consists of a single layer of flat endodermal 

 cells. The embryonic plate presents an oval surface and in section consists of two 

 or three layers of high cylindrical cells, whereas the amniotic ectoderm, in sharp 

 contrast, consists of only a single layer of flat cells. 



The Herzog (1909) ovum is about the same size as those just mentioned. Its 

 chorionic wall, however, is much folded, so that originally it was probably con- 

 siderably larger. It contains a tubular structure in the body -stalk, obviously torn 

 from the embryo in the process of preservation and thus is completely detached. 

 This epithelial tube was at first interpreted as an allantois, but, as subsequently 

 determined by Professor F. T. Lewis (1917), it is clearly an amniotic duct, no 

 allantois being present in this embryo. Apparently the yolk-sac is larger than the 

 amniotic cavity (largest diameter of yolk-sac 0.3 mm., amniotic vesicle 0.16 mm.). 



The Strahl-Beneke (1910) ovum, which the authors have published in a 

 splendid monograph, would fall between our first and second groups. The his- 

 tological appearance of its chorion differs only slightly from those just referred to. 

 In form it is more elongated, presenting a spindle shape that is not repeated in any 

 other ovum and probably should not be regarded as typical. The embryo shows 

 certain new features as evidence of a more advanced stage of development. In the 

 body-stalk there is a solid epithelial strand connecting the tip of the amniotic sac 

 with the chorionic ectoderm and apparently representing an amniotic duct. On 

 the ventral side of the yolk-sac are thickened, solid mesodermal strands which are 

 possibly predecessors of blood-vessels. These are not to be confused with the ring- 

 like arrangement of mesodermal cells which simulates vessels and may be seen in all 

 mesodermal parts of the ovum. In figure 32 (Strahl-Beneke) there is seen a clear 

 area in the ectodermal plate which suggests the transition of the cells into a canalis 

 neurentericus, although a distinct canal is not present. The authors also speak of 

 a head-process, including under that term the free mesodermal cells which, in the 

 middle line, beneath the embryonal area, lie between the ectoderm and endoderm, 



