EXPLANATION OF PLATE 7. XV 



The section is intended to illustrate the fact that the nuclei form a layer in the yolk 

 under the floor of the segmentation cavity. The roof of the segmentation cavity is 

 broken. 



Fig. ia. Portion of same blastoderm highly magnified, to shew the characters of 

 the nuclei of the yolk ri and the nuclei in the cells of the blastoderm. 



Fig. i b. Large knobbed nucleus from the same blastoderm, very highly magnified. 

 Fig. 2 c. Nucleus of yolk from the same blastoderm. 



Fig. 3. Longitudinal section of blastoderm of same stage as fig. i. (Hardened in 

 chromic acid.) 



Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2. Magnified 

 45 diameters. (Hardened in osmic acid.) 



It illustrates (i) the characters of the epiblast ; (2) the embryonic swelling; (3) 

 the segmentation cavity. 



Fig. 5. 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 diameters. 



Fig. 5 a. Section through the periphery of the embryonic rim of the blastoderm 

 of which fig. 5 represents a section. 



Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger 

 than that represented on PL 8, fig. B. 



Fig. 7. Section through the most projecting portion of the embryonic rim of a 

 blastoderm of the same age as that represented on PL 8, fig. B. The section is drawn 

 on a very considerably smaller scale than that on fig. 5. It is intended to illustrate 

 the growth of the embryonic rim and the disappearance of the segmentation cavity. 



Fig. 7 a. Section through peripheral portion of the embryonic rim of the same 

 blastoderm, highly magnified. It specially illustrates the formation of a cell (c) 

 around a nucleus in the yolk. The nuclei of the blastoderm have been inaccurately 

 rendered by the artist. 



Figs. 8 a, 8b, 8c. Three sections of the same embryo. Inserted mainly to illus- 

 trate the formation of the mesoblast as two independent lateral masses of cells ; only 

 half of each section is represented. 8 a is the most posterior of the three sections. 

 In it the mesoblast forms a large mass on each side, imperfectly separated from the 

 hypoblast. In 8 b, from the anterior part of the embryo, the main mass of mesoblast 

 is far smaller, and only forms a cap to the hypoblast at the highest point of the 

 medullary fold. In 8 c a cap of mesoblast is present, similar to that in 8 b, though 

 much smaller. The sections of these embryos were somewhat oblique, and it has 

 unfortunately happened that while in 8 a one side is represented, in 8 b and 8 c the 

 other side is figured, had it not been for this the sections 8 b and 8 c would have been 

 considerably longer than 8 a. 



Fig. 9. Longitudinal section of an embryo belonging to a slightly later stage 

 than B. 



This section passes through one of the medullary folds. It illustrates the continuity 

 of the hypoblast with the remaining lower layer cells of the blastoderm. 



Figs. loa, lob, loc. Three sections of the same embryo belonging to a stage 

 slightly later than B, PL 8. The space between the mesoblast and the hypoblast 

 has been made considerably too great in the figures of the three sections. 



C 2 



