211 



An extreme case is shown in t'igure 2 of which Figure 8 is an en- 

 largement. 



Price ^) speaking of the embryonic development of this form 

 states: "In all stages the eye is very small in proportion to the size 

 of the head. In stage A it is already an optic-cup, the inner or 

 retinal layer of which is several cells thick while the outer layer is 

 one cell thick." "In the youngest embryo there is a cone-shaped 

 thickening of the external epiblast the apex of which comes in contact 

 with the anterior edge of the optic-cup. This is no doubt homologous 

 with the thickening of the epiblast which in other vertebrates gives 

 rise to the lens. The fact that it does not lie in the mouth of the 

 optic-cup is to be accounted for by the fact that the eye is directed 

 slightly backwards." "In stage B there are no essential changes 

 except that there are no traces whatever of the lens." 



Dean 2) states: "In front on either side, overlapping the optic- 

 vesicles are seen dark circles which I at first mistook for lens thicken- 

 ings; they are, however, the Anlagen of marginal barbels and 

 appeared rather conspicuous at this stage" (embryo with 11 gill-slits). 



KuPFFER 3) has also observed this thickening and considers it to 

 be a rudimentary lens. If we should grant such an interpretation of 

 this ectodermal thickening, namely, that it is a rudimentary crystalline 

 lens, we should have a clear case of arrested development resulting 

 in the continuance of an embryonic condition into adult life. This 

 view is strongly supported by the condition of the retina with its 

 space of separation between the inner and outer layers of the optic- 

 cup, by the rudimentary iris, and by the often persistent choroid 

 fissure. 



I wish to acknowledge my indebtedness to Professor Frank 

 R. LiLLiE of the University of Chicago for many helpful suggestions, 

 and for the material used in this work. 



1) Q. G. Price, Some Points in the Development of a Myxinoid 

 (Bdellostoma Stouti). Verb. d. Anat. Ges., April 1896. 



2) B. Dean, On the Embryo of Bdellostoma Stouti. Festschr. z. 

 70. Geburtstag von Kabl v. Kupffer. 



3) K. V. Kupffer, Studien zur vergleichenden Entwickelungs- 

 geschichte der Kranioten. Heft 4. Zur Kopfentwickelung von Bdello- 

 stoma, München und Leipzig. 



14' 



