MUSEUM OF COMr'Ai;A.TlVE ZOOLOGY. 59 



words : '* VVenn die ziierst vorhandene hyaline Schicht eine gewisso 

 Dicke (Figg. 60 uiid 61) erreicht hat, wird dieselbe vom Hurnblatt ab- 

 gedrjingt durch eine zweite an dieses sich anbildende liyaline Schicht ; 

 m das zwischen beideu Schichten entsteheude Interstitium dringt von 

 der Peripherie her eine einzellige Lage der spindelformigen Kopfplatten- 

 elemente, die sich vorher schon in einen spitzen Winkel gegen das 

 Hornblatt am Rand der Corneaanlage gestellt batten, ein (Fig. 62) ; 

 sobald dieselben von alien Seiten her iin Pol der Cornea zusammen- 

 treffen, ist die erste hyaline Schicht von der unterdess zu der gleichen 

 Dicke entwickelten zweiten voilstandig gesondert. Ebenso wie die erste 

 durch die zweite, wird dann die zweite durch eine dritte neu sich bil- 

 dende Schicht vom Hornblatt und darauf durch eine zweite eiuwandernde 

 Lage von Kopfplattenelementen von der dritten Schicht isolirt ; diese 

 wieder vom Hornblatt durch eine vierte neue Schicht und von letzterer 

 durch eine dritte Zellenlage u. s. f." It would thus appear that a very 

 intimate connection is brought about between the ectoderm itself, the 

 relatively large portion of the substantia propria derived from it, and 

 the mesodermal elements of the cornea ; and it should be especially 

 noticed that this process goes on at a comparatively late stage of devel- 

 opment, — viz. at a time when the retinal layers are being differentiated, 

 and after the pigmented portions of the eye are well formed ; in short, at a 

 stage only a very little earlier than that at which development is arrested 

 in the eye of Typhlogobius. If such a process had ever taken place 

 here, it seems almost certain that we should see some indications of it 

 in such a stage as is shown in Figure 17 (Plate III.). But, on the con- 

 trary, what we do find is no connection between the epithelium over the 

 eye and the immediately underlying tissue, or at least almost none, and 

 no indication of a hyaline layer on the inner surface of the epithelium. 

 While, on the other hand, in older specimens (Plate II. Fig. 6) the epi- 

 dermis and the sub-epidermal tissue are in close connection, there being 

 no interruptions or spaces at all, and we have here a well defined nearly 

 structureless layer closely adherent to the epidermis. 



There is considerable individual variation in the size of the eye. In 

 three specimens, 50 mm., 60 mm., and 63 mm. long, the diameters, meas- 

 ured parallel to the long axis of the head, wei'e respectively 0.44 mm., 

 0.46 mm., and 0.47 mm. ; the diameters transverse to the long axis of the 

 head in the last two of these were, respectively, 0.39 mm. and 0.47 mm. 

 In another specimen 63 mm. long, the diameter transverse to the head 

 was 0.372 mm. ; the diameter parallel to the long axis of the head was 

 not measured in this specimen. Tliis last measurement was made on the 



