200 W. B. CHAPMAN 



the formation of the large vessels described by Patterson as 

 forming in the region where these vessels are usually formed. 



The peculiar cycle of development and retrogression through 

 which the sinus terminalis passes would indicate a further ex- 

 ample of this stage of primary differentiation persisting after 

 the time when it is usually thought to have been entirely elimi- 

 nated. It would also be interesting to know precisely how much 

 further development takes place within the embryo in the ab- 

 sence of a circulation, but this presents a problem within itself, 

 and cannot be taken up in this paper. Roux has stated that 

 this early stage of primary development persists for a consider- 

 able length of time, perhaps up until adult Ufe in the human. 

 He thinks the closure of the ductus botalli which occurs at birth 

 or a little later belongs to this stage. 



That the mechanical forces very early assert their superiority 

 in the extra-embryonic region is apparent, for with the breaking- 

 up of the anterior vitelline vein and the sinus terminalis into 

 capillaries the vessels of this region are reduced to an indifferent 

 capillary plexus. This capillary plexus continues to grow at a 

 gradually decreasing rate for a number of days after the time 

 when it should become functional. With the failure of the cir- 

 culation to start, we note, about the beginning of the third day 

 of incubation, the regressive changes already mentioned as tak- 

 ing place in the area pellucida. This process is noted in all of 

 the operated chicks, and also in the unoperated chicks that de- 

 veloped without circulation, and is plainly due to the processof 

 retraction and degeneration which has been mentioned by Thoma. 



In introducing his first histo-mechanical law, Thoma stated 

 that the surface of a vessel wall ceases to grow when the blood- 

 current acquires a definite rate. The vessel increases in size 

 when this rate is exceeded, becomes smaller when the blood- 

 stream is slowed, and disappears when it is finally arrested. The 

 breaking-up of the capillaries in the area pellucida is clearly an 

 application of the latter part of this hypothesis of Thoma's. In 

 this instance, however, there is no circulation into which the con- 

 tained blood can be pushed by the retracting capillaries, and as 

 these small vessels are filled with blood cells, when they narrow 



