Description of the NotocJwrdal Invagination. 7 



p. 238) earliest stages. Thi^ condition, in which the invagination 

 has begun to grow anteriad, would fall, according to Will's classi- 

 fication ('93, pp. 541-544), under his Stage III. 



The invagination of /3 extends anteriad about one third the 

 length of the shield, and there ends blindly. At the anterior end 

 of the invagination, cells along its floor are seen to be assuming a 

 columnar arrangement. Lining the pocket there is also a layer of 

 cornified degenerating substance, without nuclei and for the most 

 part without appearance of cytoplasm. The apex or blind end of 

 the cavity seems to terminate against a triangular mass of this 

 degenerating substance. It looks as though the cavity is extend- 

 ing itself forward at the expense, i. e. by the destruction, of cells 

 which formerly composed a solid infolding. Ventral and anterior 

 to the layer of columnar cells which lines the invaginated cavity 

 there exists another cell layer, which seems in every section to be 

 sharply separated from the former. The cells of this second layer 

 also have assumed a columnar form for a certain distance back of 

 the apex of the cavity. 



Mitsukuri ('93, p. 238) has not detected below the primitive 

 knob that independent layer of cells which, according to Wencke- 

 bach ('91) and Mehnert ('92), is continuous with the lower layer 

 of the shield, — a layer which the latter author, in agreement with 

 Kupfifer, designates as paraderm, but which Wenckebach calls 

 coenogenetic entoderm. At first glance it seems as though we 

 have here that layer which, according to Will ('92^), develops 

 later than the invaginated entoderm, and is therefore called by him 

 secondary entoderm. In the present instance, however, I find 

 myself unable to decide whether to consider this lower layer as at 

 all belonging to embryo y8. Embryo /3 is crowding its invagina- 

 tion into the wall and lumen of the already developed invagination 

 of embryo a. What I interpret as a cross section of the lumen of 

 the notochordal cavity of a (Fig. 14) is situated above and to the 

 right of the notochordal cavity of /3, which seems to be growing 

 into the entoderm of a; consequently, it is impossible to deter- 

 mine what portion of the surrounding entoderm belongs to a and 

 what to /S. Also underneath the primitive knob and streak of 

 a (Plate IV. Figs. 17-19) an entodermic layer lies upon the 

 yolk. In the posterior region of the primitive knob and the an- 

 terior portion of the streak this layer is distinctly separated from 

 the mesoderm. If we pass anteriad along the notochordal area 



