No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 391 



It is thus perfectly obvious that if this portion of the germ- 

 ring ever enters into the formation of the embryo it must do 

 so at a relatively later period than in Amphibia. The pro- 

 cess by which the greater portion of the germ-ring is converted 

 into embryo is an antero-posterior coalescence (concrescence) 

 of its lateral portions, forming the primitive streak {p.s). In 

 accepting concrescence as the method, I adopt the view of His, 

 that the head is practically a fixed point. In this diagram I 

 have also represented the head of the embryo as arising at the 

 apical pole. This is based directly upon the unpublished ob- 

 servations of Professor Whitman. The extent of the backward 

 growth of the embryo through proliferation is difficult to deter- 

 mine. I have represented it as extending from b to c, rather 

 to illustrate the fact that a certain portion is thus formed than 

 to indicate its exact proportional extent. 



Fig. 3 represents a still greater increment in the mass of 

 yolk, as compared with the blastodisc (Amiurus, Lophius, Ba- 

 trachus, and Elasmobranchs). The first outlines of the portion 

 of the embryo {ab), which is to be considered largely the result 

 of a differentiation in situ, appears at a time when the blasto- 

 disc covers a smaller portion of the yolk than in the forms 

 represented in Fig. 2, its margin being indicated by the line \b. 

 The successive positions occupied by the germ-ring and corre- 

 sponding elongation of the embryo are indicated, as in the 

 preceding, by the numerals i, 2, 3, 4. When the region indi- 

 cated by 5 is reached the ring constricts, leaving a part of the 

 embryo (6^) behind. This is due to the fact that the forma- 

 tive area is so far removed from this portion that it cannot pass 

 over the yolk before the embryo is formed. 



This peculiarity, first observed in Batrachus by Miss Clapp ('9i), 

 and which I have observed in Amiurus and Lophius, illustrates 

 the manner in which the "yolk blastopore" (Balfour) of Elas- 

 mobranchs has been formed, and offers a complete explanation 

 of its morphological significance. 



In a rare form of the Chick blastoderm, described by Whit- 

 man ('83), and which I have illustrated in Fig. 4, there was an 

 extension of the primitive streak {p.s) beyond the end of the 

 tail {d) to the marginal notch {m.n.). This condition was re- 



