EXPLANATION OF PLATE 3. xi 



It illustrates (i) the characters of the epiblast, (2) the embryonic swelling (e s), 

 (3) the segmentation cavity (sg). It should have been drawn upon the same scale as 

 fig. 4 ; the line above it represents its true length upon this scale. 



Fig. 4. Longitudinal section through a blastoderm at the time of the first appear- 

 ance of the embryonic rim, and before the formation of the medullary groove. 

 (Magnified 45 diam. ) 



It illustrates (i) the embryonic rim, (2) the continuity of epiblast and hypoblast at 

 edge of this, (3) the continual differentiation of the lower layer cells, to form, on the 

 one hand, the hypoblast, which is continuous with the epiblast, and on the other the 

 mesoblast, between this and the epiblast; (4) the segmentation cavity, whose floor of 

 cells is now completed. 



N.B. The cells at the embryonic end of the blastoderm have been made rather 

 too large. 



Fig. 5. Surface view of the blastoderm shortly after the appearance of the me- 

 dullary groove. To shew the relation of the embryo to the blastoderm. 



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

 appearance of the medullary groove. (Magnified 96 diam.) 



a. In the region of the groove. It shews (i) 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 (n a) derived from the yolk. 



b. Section in the region of the head where the medullary groove is deficient, 

 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 shews the thickening of the hypoblast to form the 

 notochord (ck f ). 



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 (k) where the medullary groove is deficient, (2) 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 /). 



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 splanchnopleure. 



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. 



