90 PLATT. [Vol. V. 



The anterior limit of the dorsal aorta has begun to recede from 

 the point of the chorda. The irregular blood-spaces about the 

 brain and cranial ganglia have become more or less confluent, 

 and now open into the main venous branches. From the ante- 

 rior cardinals plus anterior aorta, three pairs of vessels, with 

 the irregular outlines of the primitive venous system, extend 

 towards the dorsal wall of the brain. It will be noticed that as 

 the cardinals pass the chorda each divides to reunite before 

 opening into the aortic space posterior to the chorda (aa 1 ). I 

 believe this division and reunion to be of constant occurrence at 

 this stage, but can assign no reason for it. 



Dohrn (No. 6) finds that in Pristiurus, before the artery and 

 posterior vein of a gill arch have separated from the anterior 

 vein, there are two commissures, between the anterior and pos- 

 terior veins of each arch. These he figures, giving also a later 

 stage in Scyllium, where the artery and the posterior vein have 

 separated from the anterior vein and from one another, the 

 posterior vein having now united with the anterior vein of the 

 following arch. In this later stage but one vein commissure 

 exists in each arch. In Acanthias, at a stage between the two 

 above described, when the artery and the posterior vein have 

 separated from the anterior vein (except in the fourth gill arch), 

 but when the posterior vein is not connected with the vein of 

 the following arch, I find at least five vein commissures in both 

 the first and second branchial arches, and at least four vein 

 commissures in the third and fourth arches. The more or less 

 complete division of some of these commissures points to the 

 possible existence of seven or more commissures in each arch, 

 while the fact that many sections show the commissures to be 

 directly continued, without perceptible change in diameter, into 

 the veins of the long external gill filaments, suggests that there 

 may once have been as many commissures as filaments. 



The vessel which unites the hyoid vein with the first aortic 

 arch (mandibular) has been homologized by Dohrn with the vein 

 commissure of the true gill arches, and he considers the position 

 of this vessel decisive in determining the serial homologies of 

 the mesodermic structures which it parts. Now, in Acanthias, 

 the final vein commissures of the gill arches must represent five 

 or seven, or possibly more, original commissures, extending in 

 all over the entire curve of the gill vein from the point where, in 



