;7S 



LOCY. 



DESCRIPTIONS OF THE FIGURES. 



Figs. A, B, C, D, and E represent sections through the head region of five 

 Acanthias embryos of different ages ; A being the youngest stage, B the next 

 older, and so on. 



Figs. A', B', C, and E' represent sections through the cervical region of the 

 same embryos. D' is not represented m this series, but in place of it the section 

 C" is inserted. C" is a section of embryo C made much further back in the body 



than C. 



7nf., medullary fold; cf., "concrescence" or median furrow; mg., space enclosed 

 by the medullary folds, corresponds to the medullary groove of other vertebrates. 

 It should be carefully noted that the space indicated by nig. is an entirely different 

 formation from the median furrow {cf. in Fig. A). 



Fig. D" represents a section of embryo D, in the depressed anterior region just 

 after the head-folds have begun to grow upwards. 



Fig. F represents a section through the middle of the cephalic plate after the 

 head-folds hav begun to grow upwards, and have reached a position a little above 

 the horizontal plane. 



Fig. G shows a section through the region just back of the cephalic plate, 

 where the medullary folds are abruptly bent downwards. 



All drawings of the sections were outlined with the aid of the camera, and all 

 are magnified about 75 diameters — Zeiss A A, 2 oc. 



Figs, i, 2, 3, 4, 5, and 6 are to show especially the transformation of the frontal 

 region into the cephalic plate and certain stages in the growth of the latter. The 

 figures are taken from photographs, and all are magnified about 20 diameters. 



Fig. I. An embryo showing the head region divided into'a central and two 

 lateral parts. 



Fig. 2. Older embryo showing considerable e.xpansion of the lateral parts (L) 

 of the head region — these are expanded folds ventrally curved. In the central 

 part the tongue-like process T is conspicuous, and is in connection with the ante- 

 rior tip of the head. At the point b is shown the place where the medullary folds 

 are so abruptly bent downwards. 



Fig. 3. Older stage showing the beginning of the infolding to form the in- 

 fundibulum and the optic vesicles. The tongue-like process (T) is separated by 

 this infolding from the anterior tip of the head (T). At op., the beginning of the 

 optic vesicles is indicated ; x, one of the two posterior eminences marking the 

 posterior limit of the cephalic plate. 



Fig. 4. A still older stage showing the point mentioned for the previous figure. 

 It also shows the concentric elevations (i, 2, 3) mentioned in the text. 



Fig. 5. Stage in which the medullary folds have begun to rise. The frontal 

 flexure is considerably advanced. At op' are shown the external elevations of the 

 optic vesicles. 



Fig. 6. Embryo viewed from such a position as to show the optic vesicles 

 externally {op') and internally {op). The tongue-like process is bent downwards 

 and backwards in the median plane, and is hidden from view by an edge of the 

 head-folds. 



