360 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



TROPHOBLA S T CELLS 



EPIDERMAL 



ECTODERM 



NEURAL ECTODERM 

 NOTO CHORD 

 PRE-CHORDAL PLATE 



BLASTOCOEL HYPOBLAST 



Fig. 173. Presumptive organ-forming areas in the chick blastoderm. (A) Slightly 

 modified from Spratt, '46. (B) Schematic section of early chick blastoderm passing 

 through antero-posterior median axis. 



tion from the midline of the future embryonic axis. Anterior to the neural 

 crescent is the presumptive epidermal crescent. Within the area opaca is 

 found potential blood-vessel and blood-cell-forming tissue, as well as the 

 extensive extra-embryonic-tissue materials. 



The above description of the presumptive organ-forming areas pertains to 

 the avian blastula just previous to the inward migrations of the notochordal, 

 pre-chordal plate, and mesodermal areas; that is, just previous to the ap- 

 pearance of the primitive streak and the gastrulative process. 



4. Primary and Secondary Reptilian Blastulae 



The primary blastula of turtle, snake, and lizard embryos is akin in essen- 

 tial features to that of birds. It consists of a central blastoderm or area 

 pellucida, overlying a primary blastocoelic cavity, and a more distally situ- 

 ated opaque blastoderm, together with an indefinite periblast syncytium. A 

 localized region of the central blastoderm, situated along the midline of the 

 future embryonic axis and eccentrically placed toward the caudal end, is 

 known as the embryonic shield. 



A specialized, posterior portion of the embryonic shield, in which the upper 

 layer (epiblasi) is not separated from the underlying cells (hypoblast), is 

 known as the primitive plate (fig. 174A-D). (Consult also Will, 1892, for 



