TYPES OF CHORDATE BLASTULAE 



367 



EP I BLAST 



ENTODERM OR 

 PRIMARY 

 HYPOBLAST 



NEURAL ECTODERM 



NOTO CHORD 



ENTODERM 

 DORSAL 8LAST0P0RAL LIP 



Fig. 180. Presumptive organ-forming areas of the teleost fish blastoderm. (A) 

 Median section through the late blastoderm of Fundidus heteroclitus just previous to 

 gastrulation. Somewhat schematized from the author's sections. Presumptive entoderm 

 or hypoblast is shown exposed to the surface at the caudal end of the blastoderm and, 

 therefore, follows the conditions shown in (B). (B) Presumptive organ-forming areas 

 of the blastoderm of Fiindulus heteroclitus. Arrows show the direction of cell move- 

 ments during gastrulation. (Modified from diagram by Oppenheimer, '36.) 



appears. Within the inner cell mass, two types of cells are present, 

 namely, formative and trophoblast (figs. 177B; 178A). 



(2) Unlike that of the marsupial mammal, an overlying layer of tropho- 

 blast cells, covering the layer of formative cells, always is present (fig. 

 177B). In some cases (rabbit, pig, and cat) they degenerate (the 

 cells of Rauber, fig. 177C), while in others (man, rat, and monkey) 

 the overlying cells remain and increase in number (fig. 178A-E). 



(3) The entodermal cells arise by a separation (delamination) of cells 

 from the lower aspect of the inner cell mass (figs. 177C; 178A), 

 with the exception of the armadillo where their origin is similar to 

 that of marsupials. With these differences, the same essential goal 

 arrived at in the marsupial mammals is achieved, namely, a bilaminar, 

 formative area, the embryonic disc, composed of epiblast and hypo- 

 blast layers (figs. 177D; 178D, E), which ultimately gives origin to 

 the embryonic body. A bilaminar, extra-embryonic, trophoblast area, 

 consisting of extra-embryonic entoderm and ectoderm, also is formed 

 (figs. 177D; 178D, E). The secondary blastocoel originates between 

 the epiblast and hypoblast of the embryonic disc, while below the 

 hypoblast layer is the archenteric space (fig. 178E). 



