STAGES B TO G. THE NOTOCHORD. 313 



as to whether it is primitively a hypoblastic or a mesoblastic 

 organ ; but, from whatever layer it has its source, an excellent 

 example will be afforded of an organ changing from the layer in 

 which it was originally developed into another distinct layer. 



EXPLANATION OF PLATE 10. 

 COMPLETE LIST OF REFERENCE LETTERS. 



a/. Alimentary canal, ch. Chorda dorsalis or notochord. ch'. Ridge of hypoblast, 

 which will become separated off as the notochord. ep. Epiblast. Ay. Hypoblast. 

 //. Coalesced lateral and vertebral plate of mesoblast. m g. Medullary groove. 

 . Nucleus of yolk. n a. Cells formed around the nuclei of the yolk to enter into the 

 ventral wall of the alimentary canal, n c. Neural or medullary canal. / v. Proto- 

 vertebra. so. Somatopleure. sp. Splanchnopleure. t s. Mesoblast of tail-swelling. 

 yk. Yolk-spherules. 



Figs, i a, i b, i 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 (i) The different characters of the medullary groove in the 

 different regions of the embryo. (2) The structure of the coalesced lateral and verte- 

 bral 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 (i 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. 



Figs. 2 a, ib, ic. 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 the layers during the stage, more especially the 

 differentiation of somatic and splanchnic layers of the mesoblast. 



Figs. 3 a, 3 b, 3 c, 3 d, 3 e, $f. Sections of the same embryo as fig. 3 ( x 60 dia- 

 meters). Fig. 3 belongs to part of the embryo intermediate between figs. 3^ and %f. 



The sections shew the features of various parts of the embryo. Figs. 30, 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 the 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 commencing closure 

 of the medullary groove. 



B. 21 



