EXPLANATION OF PLATE IX. 



ep, epiblast. liy. hypoblast. 



I p. coalesced lateral aucl vertebral plate of mesoblast. 



w^ g, medullary groove. n c. neural or medullary canal. 



2? V. protovertebra. so. somatopleure. 



sp. splanchnopleure. t s. mesoblast of tail-swelling. 



ck. chorda dorsalis or notochord. 



ch\ ridge of hypoblast, which will become separated oflf as the noto- 

 chord. 



al. alimentary canal. 



7h a. cells formed around the nuclei of the yolk to enter into the ventral 

 wall of the alimentary canal. 



71. nucleus of yolk. yk. yolk spherules. 



Fig. I a. 1 b. 1 c. Three sections from the same embryo belonging to a 

 stage intermediate between B and C, of which fig. I a is the most anterior. 

 X 96 diameters. 



The sections illustrate (1) The different characters of the medullary 

 groove in the different regions of the embryo. (2) The structure of the 

 coalesced lateral and vertebral plates. (3) The mode of formation of the 

 notochord as a thickening of the hypoblast {ch'), which eventually becomes 

 separated from the hypoblast as an elliptical rod (1 a. ch). 



Fig. 2. Section through the anterior part of an embryo belonging to 

 stage C. The section is mainly intended to illustrate the formation of the 

 ventral wall of the alimentary canal from cells formed around the nuclei of 

 the yolk. It also shews the shallowness of the medullary groove in the 

 anterior part of the body. 



Fig. 2 a. 2 b. 2 c. Three sections from the same embryo as fig. 2 Fig. 2 a 

 is the most anterior of the three sections and is taken through a point 

 shortly in front of fig. 2. The figures illustrate the general features of an 

 embryo of stage C, more especially the complete closing of the alimentary 

 canal in front and the triangular section which it there presents. 



Fig. 3. Section through the posterior part of an embryo belonging to 

 stage D. X 86 diameters. 



It shews the general features of layer during the stage, more especially 

 the differentiation of somatic and splanchnic layers of the mesoblast. 



Fig. Sa.Sh.2c. Z d. 3 e. 3/ Sections of the same embryo as fig. 3 ( x 60 

 diameters). Fig. 3 belongs to part of the embryo intermediate between fig. 

 3 e and 3/ 



The sections shew the features of various parts of the embryo. Fig. 3 a. 

 3 b. and 3 c. belong to the head, and special attention should be paid to 

 the presence of a cavity in the mesoblast in 3 b and to ventral curvature of 

 the medullary folds. 



Fig. 3 d belongs to the neck, fig. 3 e to the back, and fig. 3/ to the tail. 



Fig. 4. Section through the region of the tail at the commencement of 

 stage F. X 60 diameters. 



The section shews the character of the tail-swellings and the com- 

 mencing closure of the medullary groove. 



Fig. 5. Transverse section through the anterior part of the head of an 

 embryo belonging to stage F. ( x 60 diameters). It shews (1) the ventral 

 curvature of the medullary folds next the head. (2) The absence of 

 mesoblast in the anterior part of the head, hy points to the extreme front 

 end of the aliuientarv canal. 



