144 



period it is possible to distin guish several large vessels, derived 

 from the umbilical artery and passing straight through to the 

 outer limit of the allantoic portion of the placenta, frora the 

 small capillaries which ultimately pass into the branches, on the 

 opposite side, of the umbilical vein (Figs. 5, 6, 11). I am unable 

 to speak with greater exactitude than this of the relation of the 

 various embryonic blood-vessels in the placenta to the umbilical 

 artery and umbilical vein respectively, since l have only studied 

 their arrangement by the aid of serial sections, not by means of 

 injection. 



We may now turn to the consideration of the trophoblast, by 

 which these villi are covered. In the lowest layer of the cytotropho- 

 blast the cells have the appearance of a columnar epithelium which 

 is not pierced, but so to speak, invaginated by the invading villi. 

 Passing further upwards, the villi are found covered by the syn- 

 cytial trophoblast. This forms in the earlier phases of this period, 

 a fairly thick layer, with nuclei disposed in several rows, between 

 the villi and the lacunae; but with the continued ramification 

 and elongation of the villi, it becomes progressively thinner, 

 until finally the nuclei are disposed in a single row. The nuclei, 

 which are large, and have the character already described, project, 

 covered, of course, by a thin sheet of cytoplasm, into the lumina 

 of the lacunae (Fig. 36). 



The cytoplasm of the trophoblast may be shown to contain 

 fat, and sometimes other granules as well. In addition to the 

 trophoblast which immediately covers the villi there are also to 

 be found distributed throughout this portion of the placenta 

 oblong patches of cells which are not penetrated by allantoic 

 vessels. These patches are elongated in the same direction as the 

 villi themselves, and are derived from tracts of the cytoblast, which, 

 as explained above, are carried upwards on the first penetration 

 of the allantois; later they will be perforated by foetal capillaries, 

 and, as isolated patches, will disappear. 



The lacunae of the trophoblast are exceedingly numerous and 

 for the most part small; they are irregular in outline, and are 



