ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 283 



does not prevent or apparently greatly retard ovulation ; increased 

 internal pressure may therefore be the most important factor in normal 

 ovulation ; yolks of partly formed and fully formed eggs may be 

 absorbed rapidly and in large numbers from the peritoneal surface 

 without causing any serious derangement of normal metabolic processes. 



Development of Lymphatics in Chick Embryo.*— Eleanor L. 

 dark finds that the primary superficial lymphatics of chick embryos 

 form a rapidly growing, frequently anastomosing capillary network, 

 with numerous open connexions with the venous system. For over 

 twenty-four hours the pressure in these earliest lymphatics remains 

 less than the side-pressure in the connecting veins, and, consequently, 

 there is no lymph-fiow in the early plexus. Instead, it contains blood 

 Avhich backs up into it from the communicating veins. 



The pressure of the fluid in the lymphatics gradually increases and 

 finally overcomes that of the veins. The first lymph-flow which is then 

 established is feeble and easily disturbed. It gradually l)ecomes more 

 rapid and steady, but its course is easily altered by various mechanical 

 factors. A day later, the pressure in the superficial lymphatics has 

 increased still more, while, for various reasons, the outflow into the 

 veins is interfered with. At certain points two conflicting pressures are 

 present, and here the lymph-flow becomes sluggish. 



The endothelium of these early lymphatics responds to the passage 

 of fluid over its interior by the differentiation of definite ducts or 

 channels out of the indifferent primitive network. With the increased 

 flow of lymph these channels enlarge and more channels form. That 

 the formation is due to the lymph-flow and to mechanical factors rather 

 than to arbitrary predetermination is evident from the frequent varia- 

 tions which occur in the position of the main ducts in chicks of the 

 same stage. The endothelial wall of the early lymphatics also responds 

 to the increased pressure caused by interference with the lymph-flow 

 and the damming back of fluid, by expanding to form sac-like enlarge- 

 ments. The size which these sacs may attain is influenced to some 

 extent by the looseness of the surrounding tissue. The general result 

 of the study is that lymphatic endothelium reacts to the pressure and 

 flow of the fluid inside the walls of the vessels, and that the formation 

 of lymph trunks and of lymph sacs from a primitive plexus represents a 

 response to such stimuli. 



Influence of Exercise on Growth of Rat.f — Shinkishi Hatai has 

 experimented with the albino rat in order to discover the effect of long- 

 continued exercise. Long-continued exercise, 90-180 days in revolving 

 cages ("equivalent to a period of 7-14 years in man"), produces 

 many striking modifications. The heart, kidneys and liver show an 

 average excess of about 20 p.c, while the spleen shows a similar amount 

 of deficiency. The brain-weight shows an average excess of 4 p.c, 

 while no change was observed in the spinal cord. The ovaries showed 



* Amer. Jouru. Anat., xviii. (1915) pp. 399-440 (9 figs.). 

 t Anat. Record, ix. (1915) pp. 647-66. 



