No. 2.] DEVELOPMENT OF PETROMYZON: 293 



capsule, which Balfour (5) with KolHker and others regard as 

 " a cuticular membrane deposited by the epithehal cells of the 

 lens;" a view which is mainly supported by the fact that "at the 

 time when the lens-capsule first appears, there are no mesoblast 

 cells to give rise to it." Whatever may be the fact with regard 

 to the formation of this structure in other vertebrates, there can 

 be no doubt that in Pctromyzon a layer of mesoblastic cells is 

 included in the eye between the retina and the lens; and this, 

 taken in connection with Miiller's statement that he has found 

 cellular elements in the lens-capsule, would certainly seem to 

 indicate that in this type at least the capsule is of mesoblastic 

 origin. 



The changes which take place in the optic vesicle itself dur- 

 ing larval life are not of a radical character. In the smaller 

 larvae the eye is so minute that it is rather difficult to find. The 

 anterior wall of the retina increases slowly in thickness, and 

 several layers of cells make their appearance, which, however, 

 are not at all clearly differentiated from each other, while the 

 posterior wall becomes flatter, and in Ammocoetes of 20 mm. 

 length is pigmented, thus forming the pigmented epithelium of 

 the choroid. The various portions of the retina are very im- 

 perfectly developed in the larvae, the rudiments of the rods and 

 cones appearing but a very short time before the metamor- 

 phosis, and, as Langerhans has remarked, the blindness of the 

 Ammocoete is caused not only by the deep situation of the eye 

 and the opacity of the overlying skin, but also by the absence 

 of percipient elements. 



The mesoblastic envelops of the eye are at first very simple. 

 A mass of mesoblastic cells in front of the lens (Fig. 27, PI. X) 

 arranges itself into a layer, the membrane of Descemet, which 

 is imbedded in a layer of fibrous connective tissue, which sur- 

 rounds the optic vesicle and represents the choroid ; this layer 

 is pigmented, A cornea cannot be said to be present in the 

 larva, a thick layer of dermis separating the optic vesicle from 

 the epidermis, which, over the eye, is entirely indifferent and 

 like the general epidermis. Nor have I detected any begin- 

 nings of the formation of the iris beyond a slight thinning of 

 the exterior edges of the optic cup. After the first formation 

 of the lens, and in small larvae, the eye lies in close contact with 

 the epidermis (Fig. 26, PI. X) ; the formation of the dermis 



