EXPLANATION OF PLATES VII, VIII & IX, 



Illustrating Messrs. Balfour and Deigliton^s Paper on '' A 

 Renewed Study of the Germinal Layers of the Chick/^ 



N.B. — The series of sections are in all cases numbered from before 



backwards. 



List of Reference Letters. 



ep. Epiblast. ht/. Hypoblast, m. Mesoblast. gr. Germinal wall. 

 yk. Yolk of germinal wall, pv s. Primitive streak, pr. g. Primitive 

 groove, ch. Notochord. a. p. Area peliucida. o. p. Area opaca. 



PLATE VII. 



Series A, 1 and 2. — Sections through the blastoderm before the appearance 

 of primitive streak. 



1. Section through anterior part of area peliucida in front of em- 

 bryonic shield. The hypoblast here forms an imperfect layer. The 

 figure represents about half the section. 2. Section through same blas- 

 toderm, in the region of the embryonic shield. Between the epiblast 

 and hypoblast are a number of undifferentiated cells. The figure repre- 

 sents considerably more than half the section. 



Series B, 1, 2 and 3. — Sections through a blastoderm with a very young 

 primitive streak. 



1. Section through the anterior part of the area peliucida in front 

 of the primitive streak. 2. Section through about the middle of the 

 primitive streak. 3. Section through the posterior part of the primi- 

 tive streak. 



Series C, 1 and 2. — Sections through a blastoderm with a young primitive 

 streak. 



1. Section through the front end of the primitive streak. 2. Section 

 through the primitive streak, somewhat behind 1. Both figures 

 show very clearly the difference in character between the cells of the 

 epiblastic mesoblast of the primitive streak, and the more granular 

 cells of the mesoblast derived from tlie hypoblast, 



fiG. D. — Longitudinal section through the axial line of the primitive 

 streak, and the part of the blastoderm in front of it, of an embryo 

 duck with a well-developed primitive streak. 



Series E,* 1, 2, 3 and 4.— Sections through blastoderm with a primitive 

 streak, towards the end of the first stage. 



1. Section through the anterior part of the area peliucida. 2. Sec* 

 tion a little way behind 1 showing a forward growth of mesoblast from 

 the primitive streak. 3. Section through primitive streak. 4. Section 

 through posterior part of primitive streak, showing the great widening 

 of primitive streak behind. 



^ 3 and 4 of this series are placed on Plate Vlll. 



