352 



the blastodisc, or germ-ring, and represents the extent of primitive 

 streak. The numerals i, ^, 5, 4 represent successive positions occupied 

 by the germ-ring in its passage over the yolk. 



Fig. 29 represents the condition observed in Necturus at the time 

 the embryo appears. The blastodisc has not extended as far over the 

 yolk as in the other Amphibia. One might say that the embryo has 

 reached the same stage of differentiation at a relatively earlier period. 

 That portion of the embryo which is to be regarded as having formed 

 in situ extends from ^. to i?. The portion formed through backward 

 extension by crossed lines, while that formed through concrescence 

 is represented by the primitive streak P. S. When compared with the 

 preceding diagram we see that the portion of the embryo formed 

 through concrescence is increased while differentiation in situ plays 

 a decreasing role. In a previous discussion of the tectonics of embryo 

 formation I regarded the upper pole of the egg as the anläge of the 

 embryo. The experiments on Necturus, however, lead me to infer that 

 the head may form in an area considerably removed from the upper pole. 



Fig. 30 illustrates the condition observed in the majority of Teleosts. 

 There is a still greater proportional increase in the mass of yolk and 

 as a result we observe that the anterior portion of the embryo between 

 Ä. and B. reaches the same relative stage of differentiation as Nec- 

 turus at a time when the blastodisc has extended over about one-third 

 of the yolk. The extent of the embryo formed through differentiation 

 in situ is thereby further reduced. In these forms the process of 

 infolding at the dorsal lip plays a less important role than in the 

 Amphibia while overgrowth becomes a more important factor. It may 

 be remarked that the most active or formative tract Ä.-B. is farther 

 removed from that portion of the blastodisc at 1 which is destined to 

 form the caudal end of the embryo. It is thus obvious that if this 

 portion of the blastodisc enters into the formation of the embryo it 

 must do so at a relatively later period than in the Amphibia. The 

 process through which the greater portion of the germ-ring, or thickened 

 blastodermic margin, is converted into embryo is an antero-posterior 

 coalescence or concrescence of its lateral margins, thus giving rise to 

 the greatly extended primitive streak {I: S.). 



The final result of this continued increase in the amount of yolk 

 material is shown in Fig. 31 and represents the condition observed 

 in certain Teleosts such as Batrachus, the Elasmobranchs and Aves. 

 In these forms the portion of the embryo (A.-B.) which differentiates 

 in situ is farther decreased and appears at a time when the blastodisc 

 covers a much smaller portion of the yolk. The portion of the blasto- 



