ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 159 



matous tissue in the left dorso-lateml portion of the mesentery. In the 

 first stages of its development it is impossible to make a sharp distinc- 

 tion between the ccelomic epithelium and the mesenchyme of the 

 mesogastrium. Proliferating activity on the part of the epithelial cells 

 may result in the crowding of some of the daughter cells into the 

 underlying mesenchyme. After the splenic rudiment is established 

 (15 mm. pig), the peritoneal cells are sharply separated from the 

 mesenchyme by a distinct limiting membrane. The earliest splenic 

 vessels are branches of the mesenteric artery, which form a network. 

 Blood-forming activity in the spleen begins in pig embryos of 8 to 4 cm. 

 in length ; cells of the mesenchyme become free lymphocytes. Erythro- 

 poietic activity is seen first in 4 to 6 cm. pig embryos. Erythroblasts 

 are seen in small graups in lacunte that were formed by the first free 

 cells as they were cut off from the mesenchymal syncytium. The 

 erythropoiesis is largely extra-vascular. Granulopoiesis is very limited 

 in the pig's spleen at any stage. The authors discuss the differentiation 

 of " white pulp," the lymphoid or follicular portion of the spleen, and 

 the appearance of large lymphocytes. J. A. T. 



Development of Opossum's Stomach and Intestine. — C. H. 

 Heuser {Amer. Journ. Anat., 1921, 28, 341-6i), 6 pis.). As the intra- 

 uterine period in the opossum is very brief, the intestinal nutrition 

 commences in the new-born animals while they are relatively very 

 immature. Ten days after the beginning of segmentation, or thirteen 

 days after insemination, the young opossums appear in the pouch ; they 

 become at once attached to the teats, and milk soon enters their 

 stomachs. Five days before birth, however, the digestive tract of the 

 opossum is no further advanced than this system in the three-day chick 

 embryo ; the foregut extends forward from the broad connexion with 

 the yolk-sac, and the first rudiments of the liver, hind-gut and allantois 

 are just appearing. The transformation brought about during these 

 last five days is remarkable in its rapidity. Certain of the changes, 

 such as the enormous dilatation of the stomach, must be largely due to 

 mechanical causes ; other alterations, as in the formation of villi and 

 the differentiation of the epithelial cells in the upper parts of the small 

 intestine, must be accounted for by the natural processes of development 

 of the embryo, aided or hastened possibly by the early functional 

 requirements of the organs. J. A. T. 



Hypertrophy of Remaining Ovary after Semispaying. — Hayato 

 Arai {Amer. Journ. Anat., 1920, 27, 59-79). Semispaying seems to 

 have no influence on the growth of the body of the albino rat. There 

 is compensatory hypertrophy of the remaining ovary, independent of 

 coitus or pregnancy. The number of ova in the right ovary, excised at 

 twenty days, and in the left surviving ovary in the same rat are about 

 the same, despite the fact that the weight of the surviving ovary is two 

 to ten times greater than that of the right ovary at the time of removal. 

 On the other hand, the number of ova in the surviving ovary is 

 12 p.c. less than that in the ovary of the corresponding side of the 

 controls in the same litter, and at the same ages ; nevertheless, the 

 surviving ovary weighs one and a half to two times more than the 



