424 



original function in the early Ichthyopsida, which would account for 

 its persistence in the embryos of recent Ichthyopsida, 



Note upon the Occurrence of the Epibranchial Groove 

 in Amniote Embryos. 

 In the embryos of certain Amniotes a vestigial structure shows 

 itself which is strongly suggestive of the epibranchial groove. 



Prenant (36 and 37) described as hypochordal a transient outgrowth 

 of the dorsal pharyngeal hypoblast in the region corresponding to the 

 branchial region in certain lizards and snakes. Its form varied some- 

 what according to the species, and thus it was tnbular with a lumen 

 only in its distal portion, or it was an open groove, or an open groove 

 along part of its length and a band along the remainder, or it was a 

 band alone. This band was composed of a double layer of cells and 

 resembled a very deep groove with apposed walls. The outgrowth 

 cannot be compared with the hj^pochord, for it was confined to the 

 branchial region, it did not reach to the notochord, and it did not 

 separate from the hypoblast. A very important fact is that it arose 

 at about the same time as the thyroid, a fact which Prenant casually 

 noticed, but of which he did not grasp the significance. Having com- 

 pared it with the hypochord, and accepted Klaatsch's suggested homo- 

 logy between hypochord and epibranchial groove, he compared his out- 

 growth in turn with the latter. What is probably the right destination 

 was thus reached by the wrong road. 



Prenant collected the following references. Meuron (29) found 

 the outgrowth in embryos of Lacerta viridis. Balfour (3) discovered 

 in the head of the chick a small heap of cells upon the hypoblast and 

 against the notochord. He thought it was hypochordal ; but it was 

 close to the thyroid region and in an embryo of about the age at 

 which the thyroid appears. Balfour in turn mentioned that Marshall 

 found the same in the chick (27). Prenant referred also to a figure 

 in a paper by Saint-Remt in which two processes are shown upon the 

 dorsal phar3'ngeal hypoblast in the fowl or duck (41, his Pig. 11). 

 He drew attention to a statement by Keibel regarding a temporary 

 union maintained in dog embryos between notochord and hypoblast, 

 somewhat behind the anterior end of the gut, by a band of cells (19). 

 A paper by Platt is cited under the impression that Necturus is a 

 reptile (35). 



Wheeler also (48), commenting upon Prenant's paper, has denied 

 the hypochordal nature of the outgrowth and has admitted its homo- 

 logy with the epibranchial groove. 



Nicolas (31), in the embryos of certain birds, observed the same 

 feature as did Balfour and Marshall. In nearly all of the figures it 

 is shown as a ridge, but in a chick embryo the top of the ridge was 

 rounded off as if about to separate ^). Comparison was made with Pre- 



1) But this roundness extended throughout only four sections. In 

 an older embryo this rounded portion was separated and was below 



