108 EDWARD PHALPS ALLIS jr. 



paration and proper examination would too much retard other work 

 I have in progress. 



The investigation covers stages of Amia ranging from an 

 embryo 6 mm long to a larva 50 mm in length. Of several 

 important stages reconstructions were made, from camera drawings, 

 but as these reconstructions were based on base lines arbitrarily as- 

 sumed, only one of them is reproduced, that of a 12 mm embryo. 

 The others seem more or less distorted, but that, naturally, does not 

 seriously affect their value in so far as the general relations and con- 

 nections of the arteries with each other are concerned. 



In 12 mm embryos of A^nia the tish has already passed the em- 

 bryonic condition, and as I am much more familiar with this larval con- 

 dition than with the earlier, embryonic ones, I shall first fully describe 

 it and then simply note the differences found in older and younger ones. 

 A series of figures are given, representing, in a purely diagrammatical 

 manner, the disposition of the arteries at several stages in their de- 

 velopment. Similar figures are given showing the arteries at three dif- 

 ferent stages of their develoi)ment in Lepidosteus, as given by Müller, 

 and in one stage of their development in Selachians, as given by Dohrn. 



The general level of the dorsal surface of the mouth cavity, in 

 12 mm larvae of Amia is, in the region of the pseudobranch, nearly 

 horizontal. The mucous, lining membrane of the lateral wall of the 

 cavity here lies immediately internal to the inner surface of the 

 pterygo-quadrate cartilage, and the lateral surface of the cavity, on 

 each side, joins the dorsal surface at an angle which is only slightly 

 larger than a right angle. This angle, formed where the two sur- 

 faces unite, forms the dorso-lateral edge of this part of the mouth 

 cavity, and here runs forward and mesially at about 45 ^ to the mid- 

 vertical plane of the body. Parallel to it, and distant from it by 

 only about one tenth of the full width of the dorsal surface of the 

 cavity, is the oral opening of what Wright (9) has called the 

 pseudobranchial chamber. This opening has two distinctly different 

 portions, an anterior portion lying directly ventral to the pseudo- 

 branch, and a posterior portion lying posterior to that organ. The 

 anterior portion is relatively large at the level of the dorsal surface 

 of the mouth cavity, but it soon contracts to a narrow slit-like opening 

 which leads directly into the pseudobranchial chamber. The posterior 

 portion is wider than the anterior one and is the ventral opening of 

 a large deep groove, or channel, with nearly parallel sides and a 

 rounded bottom. The dorsal part, or bottom, of this channel is di- 



