490 SUMMARY OF CURRENT RESEARCHES RELATING TO 



remain as a series of shallow cups, having been stretched transversely 

 by the expansion of the ob-placenta. The epithelium of the cups unites 

 into a new continuous layer, the glands grow up into follicles and are 

 again sti*etched out by the expansion of the walls. The ectoderm which 

 attaches the embryo disappears from the surface of the placenta during 

 the eleventh day ; the vascular connective tissue of the allantois probably 

 grows by forming true villi into the placenta, and so comes close to the 

 maternal circulation. 



The observations on the human subject are, as may be supposed, 

 somewhat scattered. The author linds that the umbilical cord is not 

 covered by the amnion, but by an extension of the foetal epidermis. 

 Its ccelomatic cavity is completely obliterated during the third month, 

 and a little later the stalk of the yolk-sac is resorbed. The allantoic 

 epithelium persists as a tube or cord of cells for a long period. The 

 blood-vessels have specialized walls derived from the surrounding 

 mesoderm, but have no true adventitia. Connective-tissue fibres begin 

 to develope during the third month. The amnion is covered by a single 

 layer of ectodermal cells, which are connected by conspicuous inter- 

 cellular bridges ; it has no true stomata. The chorion consists of two 

 layers, mesoderm and ectoderm, both of which are present over all parts 

 of the chorion during the entire period of pregnancy. The mesoderm 

 has at first a dense colourable matrix, with cells, which colour very 

 slightly. During the second month the matrix loses its colouring pro- 

 perty, and subsequently the cells acquire a greater affinity for colouring 

 matters. The matrix assumes a fibrous appearance, and in one region 

 the mesoderm is differentiated into an outer fibrillar layer and an inner 

 and thicker stroma-layer. During the first month the ectoderm divides 

 into an outer dense protoplasmic layer and an inner less dense cellular 

 layer. In the later stages of pregnancy the whole ectoderm of the 

 smooth chorion acquires the character of the cellular layer, except near 

 the margin of the placenta. The villi are at first of awkward and 

 irregular form, but their branching gradually becomes more regular, and 

 the twigs acquire a slender and more uniform shape. 



The menstruating uterus is characterized by hypersemia, by hyper- 

 plasia of the connective tissue of the mucosa, and by hypertrophy of 

 the uterine glands ; the upper fourth of the mucosa is loosened and 

 breaks off, but there are no decidual cells. The changes of the uterus 

 during menstruation and gestation are homologous, the menstrual cycle 

 being prolonged and modified by pregnancy ; hence it is that conception 

 takes place only at the menstrual period, for the ovum can only modify 

 the menstrual change, not initiate the formation of a decidua. No 

 satisfactory explanation of the origin of the amnion has yet been 

 offered. The placenta is an organ of the chorion, but we possess no 

 positive information as to how it performs its nutrient functions. 



Fecundation and Segmentation of Ova of Rats.* — Professor A. 

 Tafani has observed four stages in the maturation of non-fertilized 

 ova of rats. In some the maturation-spindle extends under the surface, 

 while in others it is directed towards a point of this surface, which it 

 raises up. In some one, and in others two polar globules may be 

 seen to be expelled. Fecundation takes place when the female pro- 

 nucleus is on the point of being formed. Not more than one spermatozoon 



* Arch. Ital. Biol., xi. (1889) pp. 112-7. 



