88 EDWARD MCCRADY, JR. 



Miscellaneous details. The first aortic arch is no longer 

 plexiform, and its branches near the optic lobe have become 

 organized into the ophthalmic artery. Between the first and 

 second gill pouches plexiform branches from the ventral and 

 dorsal aortae are uniting to form the second aortic arch 

 (fig. 28, reconstruction and section D). Posterior to the level 

 of the eleventh somite many small vitelline branches leave the 

 aortae and pass into the area vasculosa (sections I, J, K). 

 From this plexus the vitelline or superior mesenteric artery 

 will later be derived. There are segmental branches from 

 the aortae at all intersomitic levels. 



The area vasculosa is rich in blood islands and includes a 

 continuous plexus of small vessels connecting the sinus termi- 

 nalis with both the vitelline veins and the vitelline arteries, 

 so that dislodgement of the blood cells from the islands and 

 continuous circulation will begin as soon as the heart starts 

 beating. 



The forelimb plate has thickened into a ridge which over- 

 hangs the more lateral somatopleure. 



Comparative notes. The 9-day-old rabbit (e.g., Minot's 

 specimens 619, 568, 570) is more advanced than the opossum 

 of the same age in having its brain roofed over, and in having 

 a caudal amniotic fold. It is less advanced in number of 

 somites, in the stage of development of the optic and otic 

 anlagen and of the pharyngeal pouches. In other respects it 

 is very similar. 



A 1.77-mm. lizard (Lacerta agilis Peter's specimen no. 42) 

 is slightly ahead of the stage 26 opossum in the development 

 of the head amnion, and in having a rudiment of the allantois. 

 It is very decidedly behind the opossum in somite number, in 

 the development of the eye and ear, in the pharyngeal pouches, 

 and most of all, in the vascular and urinogenital systems, 

 which can hardly be said to have begun in the lizard. 



A 33-hour chick (10 or 12 somites Keibel's no. 19) is 

 similar in the development of the optic anlagen, and in the 

 vascular and urinogenital systems. It is more advanced in 



