EXPLANATION OF PLATES XTII, XIV & TS—Contimed. 



Fig. 6 a and b. — Two transverse sections of the same embryo, shortly 

 after the appearance of the medullary fjroove. (Magnified 96 diam.) 



a. In the region of the groove. It shows (1) the two masses of mesoblast 

 on each side, and the deficiency of the mesoblast underneath the medullary 

 groove ; (2) the commencement of the closing in of the alimentary canal 

 below, chiefly from cells (« a) derived from the yolk. 



b. Section in the region of the head where the medullary groove is de- 

 ficient, other points as above. 



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 shows the thickening of the hypoblast to 

 form the notochord (cA'). 



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 shows (1) 

 the flattened head (K) where the medullary groove is deficient, (2) the caudal 

 lobes, with a groove between them ; it also shows that at this point, the 

 medullary groove has become roofed over and converted into a canal. 



Fig. 8 ^.^Transverse section of fig. 8, passing through the line a. (Mag- 

 nified 90 diam.) The section shows (1) the absence of the medullary groove 

 in the head and the medullary folds turning down at this time instead of 

 upwards; (2) the presence of the pleuro-peritoneal cavity in the head (iJ^) ; 

 (3) the completely closed alimentary canal {a I). 



Fig. 8 b. — Transverse section of fig. 8, through the line b. (Magnified 

 90 diam.) It shows (1) the neural canal completely formed ; (2) the vertebral 

 plates of mesoblast not yet split up into somatopleure and sphanchnopleure. 



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 ofif 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 I) 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 (o p) 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 shows (1) the formation by a pinching off from the top of the alimen- 

 tary canal of a peculiar body which unr'erlies the notochord {x) ; (2) the 

 primitive extension of the pleuro-peritoneal cavity up to the top of the 

 vertebral plates. 



Fig. 11 a, b, 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. (Magnified 96 diam.) In all of these the muscle-plates are shown 

 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 shown in all the figures {m p'). 



The figures further show (1) the formation of the spinal nerves {apn) as 



