110 DEVELOPMENT OF THE ELASMORRANCH FISHES. 



Fig. 7 a and b. Two transverse sections of an embryo about the age or rather 

 younger than that represented in fig. 5. (Magnified 96 diam.) 



a. Section nearer the tail ; it shews the thickening of the hypoblast to form the 

 notochord (c/i'). 



In b the thickening has become completely separated from the hypoblast as the 

 notochord. In a the epiblast and hypoblast are continuous at the edge of the section, 

 owing to the section passing through the embryonic rim. 



Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (i) the 

 flattened head (A) where the medullary groove is deficient, (?) the caudal lobes, with 

 a groove between them; it also shews that at this point, the medullary groove has 

 become roofed over and converted into a canal. 



Fig. 8 a. Transverse section of fig. 8, passing through the line a. (Magnified 

 90 diam.) The section shews (i) the absence of the medullary groove in the head 

 and the medullary folds turning down at this time instead of upwards; (2) the pre- 

 sence of the pleuro-peritoneal cavity in the head (//); (3) the completely closed 

 alimentary canal (a I). 



Fig. 8 b. Transverse section of fig. 8, through the line b. (Magnified 90 diam.) 

 It shews (i) the neural canal completely formed ; (2) the vertebral plates of mesoblast 

 not yet split up into somatopleure and splnnchnopleure. 



Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object a 

 little older than the embryo represented in fig. 8. (Magnified 20 diam.) The internal 

 anatomy has hardly altered, with the exception of the medullary folds having closed 

 over above the head and the whole embryo having become more folded off from the 

 germ. 



The two caudal lobes, and the very marked groove between them, are seen at t s. 

 The front end of the notochord became indistinct, and I could not see its exact 

 termination. The epithelium of the alimentary canal (a 1} is seen closely underlying 

 the notochord and becoming continuous with the epiblast at the hind end of the 

 notochord. 



The first visceral cleft (i v c) and eye (of) are just commencing to be formed, and 

 the cranial flexure has just appeared. 



Fig. 10. Section through the dorsal region of an embryo somewhat older than 

 the one represented in fig. 9. (Magnified 96 diam.) 



It shews (i) the formation by a pinching off from the top of the alimentary canal of 

 a peculiar body which underlies the notochord (x) ; (2) the primitive extension of the 

 pleuro-peritoneal cavity up to the top of the vertebral plates. 



Plate 4. 



Fig. i r a, fi, and c. Three sections closely following each other from an embryo 

 in which three visceral clefts are present; a is the most anterior of the three. (Mag- 

 nified 96 diam.) In all of these the muscle-plates are shewn at m p. They have 

 become separated from the lateral plates in b and c, but are still continuous with them 

 in a. The early formed mass of muscles is also shewn in all the figures (/// />'). 



The figures further shew (i) the formation of the spinal nerves (sf n) as small 

 bodies of cells closely applied to the upper and outer edge of the neural canal. 



(2) The commencing formation of the cells which form the axial skeleton from 

 the inner (splanchnopleuric) layer of the muscle-plate. Sections f> and c are given 

 more especially to shew the mode of formation of the oviduct (o~). 



