NOTES ON BLASMOBRANCH DEVELOPMENT. 237 



its edge, which is part of the general edge of the blastoderm, 

 the ectoderm is continuous with the endoderm which forms 

 the under side of the tongue. A good idea of the appearance 

 of a transverse section through this tongue is given by fig. 

 1 b, pi. x/ of the ' Elasmobranch Fishes ' (Mem. Ed.). The 

 hinder end of the tongue is of course notched, and the notch 

 is continued forwards along the line of the groove above men- 

 tioned as occupying the centre of the medullary plate, as a 

 slit which actually completely perforates the blastoderm, so as 

 to lead into the space between the endoderm of the tongue 

 and the yolk. This is shown clearly in fig. 3, and at a later 

 stage in fig. 4. Whether this slit is due to a bilobed back- 

 ward growth of the notched portion of the embryonic rim, 

 the growth at the middle point, i. e. at the bottom of the notch, 

 ceasing — in other words, to an emphasising of the notch already 

 present — or whether it arises as a secondary perforation of the 

 medullary plate and endoderm along the line of the groove 

 before mentioned, I am unable to say ; but I think it is due to 

 the former. 



While these changes have been taking place — and I must 

 now refer back to fig. 2 — the sides of the projecting tongue 

 become bent ventralwards and towards each other until they 

 meet or nearly meet in the ventral middle line. Now two 

 important structural results, which should be noted and under- 

 stood, follow from this bending : (i) the two angles formed by 

 the junction of the edge of the blastoderm in the embryonic 

 region with the edge of the blastoderm in the non-embry- 

 onic region — the angles, one of which is marked a in fig. 3, 

 become closely approximated ventrally beneath the embryo ; 

 and (ii) a space is enclosed on the ventral side of the embryo, 

 which space is lined by endoderm, and opens ventrally to the 

 exterior through a slit formed by the contact of the ventrally 

 bent edges of the tongue, and dorsally into the neural canal 

 by the slit in the medullary plate. This space^ is the hind 



' Old edition, pi. ix, fig. 1 b. 



' A section of the tongue iu this stage in front of the neurenteric slit is 

 shown in Schwarz's fig, 16, 



