DESCRIPTION OF PLATES VI— IX— continued. 



of the area, and is formed of epiblast and hypoblast only. Posteriorly meso- 

 blast is present. Diameter of area "81 by '61 ram. (x 49 times.) 



Fig. 12. — A somewhat later stage, in which the primitive streak reaches 

 along f rds of the length of the area, and ends behind in a knob or thickening. 

 Mesoblast now extends over the whole area. Compare figs. 32 — 6. Diameter 

 of area -97 by '74 mm. (x 49 times.) 



Fig. ]3. — A still later stage. The medullary groove is now present in 

 front of the primitive streak, and at the junction of the two the dorsal opening 

 of the neurenteric canal is seen. Diameter of area "97 by 79 mm. ( x 49 

 times.) 



Fig. 43 is a section through the medullary groove of this embryo. 



Fig. 14. — Represents a blastoderm in which the medullary groove is con- 

 siderably advanced. A dark tongue-shaped prolongation extends from the 

 region of the anterior end of the primitive streak into the hind portion of the 

 medullary groove, and another tongue-shaped dark ridge extends from the 

 anterior end of the groove nearly to the edge of the blastoderm. The neu- 

 renteric canal is not seen on the surface in this preparation, but its position is 

 indicated by a slightly lighter space at the hind end of the medullary groove. 

 Diameter of area 1*15 by "69 mm. (x 49 times.) 



Fig. 15. — The hind end of the medullary groove of an embryo slightly older 

 than that drawn in fig. 14. The medullary folds now enclose the front end of 

 the primitive streak, and the neurenteric canal perforates the floor of the 

 medullary groove. In other respects the relations of the parts are the same 

 as those in fig. 14. Compare figs, on Plates VIII and IX. Diameter 

 of this area 179 by 76 mm. ( x 73 times.) 



PLATE VII. 



All the figures on this plate are magnified 385 diameters, with the exception 

 of fig. 18, which is magnified 585 times, and fig. 30, which is magnified 265 

 times. 



Fig. 16.— a surface view of a portion of the wall of a blastodermic vesicle 

 of the same size as that drawn in fig. 3. Several of the nuclei are seen to be 

 undergoing division. (Silver-nitrate preparation). 



Figs. 17, 18, 19. — Are sections through the portion of the walls of blasto- 

 dermic vesicles to which the inner mass is attached. They are all of the stage 

 shown in fig. 3. The flattened outer layer cells lie above the inner mass, 

 no hypoblast is developed, and there is no indication of the secondary cavity. 



Fig. 18 is a section through the inner mass shown in fig. 4. The embryo 

 from which fig. 17 is taken was preserved in picric acid, that of fig. ] 8 in 

 corrosive sublimate, and that of fig. 19 in silver-nitrate. 



