62 Transactions of the 



now, excited by the presence of the blood, commences its rhythmical 

 action, and forces the blood in return through the aorta, the 

 arteriw ompJiselo mesentericse, as well as through their communi- 

 cating network of vessels, back to those blood-vessels, the ven^e 

 terininales, through which first it came ; thus a regular circulation 

 is established. In the consideration of this theory, it must be borne 

 in mind that the described process is only gradual, and that the 

 entire provisional circulatory apparatus is not completed at one and 

 the same time. On the contrary, the circulation of the blood 

 probably takes place at first through a few anastomoses, and is 

 extended in proportion to the formation of new vessels. 



The formation of new coloured blood corpuscles within the 

 follicles of the umbilical vesicle probably continues until that 

 period in which those permanent organs destined for this purpose, 

 that is, the spleen and the lymphatic glands, are sufiiciently de- 

 veloped to perform their functions. This did not seem to have 

 occurred in that embryo above mentioned, of 16 mm. in length, in 

 the umbilical vesicle of which, situated between the amnion and 

 chorion, I still recognized very distinctly those hexagonal cells with 

 their mother-blood corpuscles, notwithstanding the action of the 

 chromic acid. Neither can I remember having noticed any trace of 

 a rudimentary spleen ; if any such really had been present, it must 

 have been so small as to be easily overlooked. It might therefore 

 be supposed that the umbilical vesicle of this period of development 

 still represents the sole organ, generating blood corpuscles. The 

 circulation of the blood in this case had, as yet, not extended into 

 the villi of the chorion, for the blood-vessels of the latter were not 

 sufficiently developed. 



Some weeks later, in embryos of from eight to nine weeks old, 

 I observed a considerable change to have taken place. The um- 

 bihcal vesicle is now wasting away, but the umbilical vessels, with 

 all their ramifications, are formed, in consequence of which the 

 circulation of the blood through the finer blood-vessels of the chorion 

 is established ; the blood of the embryo is therefore carried within 

 close proximity to that of the mother. The spleen and a part of 

 the lymphatic glands are also present. The former is seen of an 

 elongated form, similar to that of the pancreas, on the left half of 

 the inferior margin of the stomach, attached by the peritoneum. 

 In addition to the coloured blood corpuscles, it also contains a con- 

 siderable number of those not fully developed colourless corpuscles 

 above mentioned. But with the appearance of these organs, and 

 the gradual wasting of the umbilical vesicle, those larger mother- 

 blood corpuscles containing several embryo- corpuscles have disap- 

 peared ; the smaller ones contain never more than one of the latter 

 at one and the same time, but are, nevertheless, capable of prodacing 

 a second one, after the first has been detached. The generative 



