292 SCOTT. ' [Vol. I. 



the head generally exhibit an acceleration of development, inas- 

 much as these organs are proportionately much larger and more 

 conspicuous in the embryo than in the adult. The minuteness 

 of the embryonic and larval eye oi Pctroviyzoii is thus chiefly a 

 secondary character. 



At this stage the layer of spindle-shaped mesoblastic cells 

 which envelopes the optic vesicle is clearly shown (those cells are 

 omitted in Figs. 23 and 24, PL X, on account of the small scale 

 on which these figures are drawn). Just after hatching, the cav- 

 ity of the optic stalks becomes obliterated and fibres begin to be 

 developed in their substance (Fig. 26, PI. X). The lens is formed 

 by an invagination of the epiblast (Fig. 25, PI. X) at or about 

 the time of hatching. This figure is copied from the one given 

 by Balfour (Comp. Embryology, Vol. II, p. 410, Fig. 291), who 

 very kindly allowed me to study the preparation from which the 

 drawing was made. It represents a stage which none of my 

 sections show. The lens is gradually separated from the epi- 

 blast, and though of very minute size, completely fills up the 

 shallow optic cup. It is composed of long, cylindrical epithelial 

 cells, and encloses a central cavity which is long persistent 

 (Fig. 27, PI. X). The posterior wall of the lens is somewhat 

 thicker than the anterior, though not markedly so. In later 

 stages this difference between the two walls of the lens becomes 

 more and more evident, the posterior becoming thicker, espe- 

 cially in the middle, and the anterior more and more flattened. 

 The cells of the posterior wall gradually take on a fibrous ap- 

 pearance and obliterate the lens-cavity, while those of the 

 anterior wall remain as a thin layer of flattened epithelium. This 

 change is to be observed only in full-grown Ammoccetes. 



The optic cup remains very small, and to the slight growth of 

 its outer edge is due the rudimentary character of the choroid 

 slit; this growth is, however, sufficient to carry away the lens 

 from its contact with the retina, and thus produce a space into 

 which, as W. Miiller has shown, there grows a process of meso- 

 blastic cells through the choroid slit, and thus forms the rudi- 

 ment of the vitreous humor. In large larvae the eye-ball has 

 a rather peculiar shape, being much depressed from above 

 downwards, so that the transverse diameter considerably ex- 

 ceeds the vertical. 



There has been much dispute as to the origin of the lens- 



