364 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. 



Fig. 174, B, the cavity was perfectly continuous towards the head 

 end, while posteriorly it was still in the form of isolated vacuoles. 



The cells which form the rudiment of the dorsal aorta are from 

 their coarsely-yolked character clearly derived ultimately from the 

 primitive endoderm, but the question remains whether they are 

 derived from the definitive endoderm directly or through the iuter- 



vac. 



Fl<;. 17-1. -Portions of liori/ont;tl .sections through /'(i//>f, ,'its larvar of ittgBI '_' 1 i A ) and 

 '!',> (H) showing the rudiment of tin- aorta in loiuitinlinal M-rtion. 



A, ami ic nidiim-nt ; <<.. xaciioli-s. 



mediary of the mesoderm. Such a section ae tluit. slio\\n in Fig. 175 

 indicates thai, tin- latl'-r is the case. Tin; definitive endoderm shows 

 a perfectly sharply denned surface, clearly market 1 off from the 



aortic, rudiment, \\hili- the mesodenn <!' (he srlrniimr >n the nt-ln-r 

 hand is C(HitiiiUMiis \vi! h the aortic rudiinciit. We may sav t licivtm-c 

 \\ilh hi^h j.rohahilit y th.-il 1 he aort ! cells are derived IVoin tlie 

 sclerotom- . 



A ivinarkahlr feature h,i heen noticed in I ho development of the 



