EXPLANATION OF PLATES. 869 



the fusion of the layers at the front end of the primitive 

 streak is the universal fusion of the layers at the dorsal lip 

 of the blastopore, which is so well known in the lower verte- 

 brate types. 



EXPLANATION OF PLATES 4345. 

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



LIST OF REFERENCE LETTERS. 



a. p. Area pellucida. ep. Epiblast. ch. Notochord. gr. Germinal wall. hy. 

 Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s. 

 Primitive streak, yk. Yolk of germinal wall. 



PLATE 43. 



SERIES A, i and 2. Sections through the blastoderm before the appearance of 

 primitive streak. 



i. Section through anterior part of area pellucida in front of embryonic 

 shield. The hypoblast here forms an imperfect layer. The figure represents about 

 half the section. 2. Section through same blastoderm, in the region of the embry- 

 onic shield. Between the epiblast and hypoblast are a number of undifferentiated 

 cells. The figure represents considerably more than half the section. 



SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primi- 

 tive streak. 



i. Section through the anterior part of the area pellucida in front of the 

 primitive streak, i. Section through about the middle of the primitive streak. 

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



SERIES C, i and 2. Sections through a blastoderm with a young primitive streak, 

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

 the primitive streak, somewhat behind r. Both figures shew very clearly the differ- 

 ence in character between the cells of the epiblastic mesoblast of the primitive streak, 

 and the more granular cells of the mesoblast derived from the 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. 



PLATE 44. 



SERIES E, 1,2, 3 and 4. Sections through blastoderm with a primitive streak, 

 towards the end of the first stage. 



i. Section through the anterior part of the area pellucida. 2. Section a little 

 way behind i shewing a forward growth of mesoblast from the primitive streak. 3. 

 Section through primitive streak. 4. Section through posterior part of primitive 

 streak, shewing the great widening of primitive streak behind. 



