425 



nant's outgrowth and with the "head part" of the hypochord ; but 

 Nicolas did not commit himself to the comparison with the epibran- 

 chial groove. Included in his review of the literature is a paper by 

 His, who mentioned a process extending from the hypoblast to the 

 notochord in front of the auditory vesicles in a chick embryo, and 

 who accounted for its existence by a pulling of the notochord upon 

 the hypoblast (17). A cleft-like extension of the gut lumen extended 

 into the process. 



This thickening or ridge upon the hypoblast described in bird em- 

 bryos must be familiar to all, and the groove below must have often 

 been observed. But so much is there sometimes to suggest a mechanical 

 origin, that ignoring them is excusable. 



Maurer (28, his Fig. 15) showed, in one of his figures of a trans- 

 verse section through the same region as the newly-appeared thyroid in 

 a lizard embryo, an outgrowth of the dorsal hypoblast comparable with 

 that of Prenant. It is not mentioned in the text. 



Gregory (12) remarked that an outgrowth similar to Prenant's 

 exists in turtle embryos. 



Reinhardt (39) drew attention to a paper by Zumstein (50), who 

 found in the head of duck embryos a thickening of the dorsal hypo- 

 blast in which sometimes a canal existed, the hypoblast below being 

 grooved. 



In a skate embryo of length 21 mm, as already mentioned, I have 

 found an outgrowth of the dorsal branchial wall opposite to the newly- 

 appeared thyroid. It was tubular and was bent with the anterior end 

 forwards. Although the thyroid had already begun its development in 

 a 17 mm embryo, the difference in time is not great; and it is quite 

 possible that the outgrowth began very soon after the 17 mm stage 

 was passed. 



Miklucho-Maclay (30), in embryos of Galeus, Mustelus and Acan- 

 thias much more advanced in development than the above 21 mm 

 skate embryo, found a dorsal diverticulum of the oesophagus just in 

 front of the stomach. It was not found in Raia, Torpedo or Trygon. 

 He compared it with the air-bladder of Teleosts, and upon this view 

 a benediction was pronounced by Gegenbaur. In comparing it with 

 the diverticulum of the skate embryo, two difficulties present them- 

 selves: the difference in position, and the difference in time of devel- 

 opment. But the first is quite a familiar feature in the case of diver- 

 ticula; while, as regards the second, we have no word that Miklucho- 

 Maclay searched younger embryos without success. His failure to find 

 it in Raia is understandable if it is the same as that which I have 

 found in the branchial region, where it would not be looked for. 



A logical conclusion from all this is that the air-bladder has been 

 derived from the epibranchial groove. But unfortunately this con- 



but not against the notochord. No one seems to have noticed a stage 



with a space shown between the notochord and the top of the ridge. 



Some will consider these features indicative of the presence of the 

 hypochord in birds. 



