MUSEUM OF COMPARATIVE ZOOLOGY. 27 



oxygenating function was the one first to be transferred ; that, so long 

 as the animal lived in the water, the gills were, under all ordinary cir- 

 cumstances, the customary channel for the elimination of carbon dioxide ; 

 and that finally, during the passage from water to land, this function was 

 also imposed on the vesicular organ of the intestinal tract, which thus 

 became a lung in the fullest sense of the word. 



IV. Embryology. 



The interesting accounts of the ontogeny of Lepidosteus given by 

 A. A^assiz and by Balfour and Parker have put us in possession of 

 important data concerning vertebrate development. It is hoped that 

 a more extended study in this field will serve to answer some of the 

 questions left unsettled by them, and will afford additional results of 

 general value in discussions concerning the phylogeny of vertebrates. 



There would be some advantages in beginning the subject of the 

 development of Lepidosteus with an account of the formation and 

 growth of the ova. Although I have considerable material for a study 

 of oogenesis, it has seemed to me better, on the whole, to defer a con- 

 sideration of the topic until the close of that part of these studies which 

 deals with organogeny, and to begin the description with the ovarian 

 ovum as it exists at the time of oviposition. But I have deviated from 

 the plan in the first part of the subject, — the egg membranes, — in 

 order to give a more complete exposition of the structures which 

 envelope the mature ovum. 



1. Egg Membranes. 



The only account of the structure of the egg membranes of Lepi- 

 dosteus is that given by Balfour and Parker ('81, p. 112, and '82, 

 p. 3G2 and foot-note). 1 It is as follows : " They [the ova of Lepi- 



i Ryder ('85, p. 146) has since given the following brief account : "In the ova of 

 Ganoids, Amia and Lepidosteus, the zona is composed, in the first instance at least, 

 of short, parallel, elastic fibers disposed in a plane vertical to that of the mem- 

 brane, these fibers being fused at their ends or just below the inner and outer 

 surfaces of the membrane. Sections through the egg membrane of Lepidosteus 

 seem to indicate the same condition of things as in Amia, in fact Dr. E. L. Mark 

 of Cambridge, Mass. has kindly shown me drawings which show the fibers of the 

 zona of the former isolated in the same condition as I have been able to separate 

 those forming the egg membrane of the latter." 



It will be seen by the following description that I do not agree with all that 



