Gastnilation in the Pigeon's Egg. 91 



together with the fact that this form is a more generalized type, 

 would seem to indicate that the development of the avian egg ought 

 more nearly to approach that of the selachian than that of the 

 teleostean ovum, and so far as the thinning-out is concerned, it does; 

 but as regards the involution of the margin it more closely resembles 

 the teleostean type. Thus, Agassiz and \Vhitman ('84) state that 

 in Ctenolabrus there is a "plain rolling under, or involution, as an 

 initiatory step in the formation of the ring." However, they regard 

 it more correct to describe the process "as an ingrowth, due both to 

 a rapid multiplication of cells, and also to the centrifugal expansion 

 of the ectoderm. "^^ The ingrowing under layer in the pigeon's 

 blastoderm with its free inner edge is in many respects comparable 

 to the "ring" in the teleostean blastoderm, and is, therefore, to be 

 regarded as a highly modified germ-ring. It is, of course, only a 

 partial ring, in that but a small part (at most an arc of 70-80 

 degrees) of the margin invaginates, while in the ordinary teleost 

 an invagination occurs around the entire margin. In the egg of 

 the Toad-fish (Batrachus tau), however, we have an interesting modi- 

 fication of the germ-ring, a condition which can be understood best 

 by quoting a part of the summary of Miss Wallace ('99), who has 

 described the development of this ring. She writes as follows: "In 

 the egg of Batrachus there is a centripetal growth of cells at the 

 embryonic pole, the ingrowth having a voluted outline in sections. 

 Around the remainder of the blastoderm there is not even the appear- 

 ance of an invagination, but only a slight thickening due to an in- 

 growth of cells from the ectoderm, and a few loose cells which may 

 represent a true germ-ring found as a layer in ordinary forms. The 

 peripheral thickening gradually fades out, first at the anterior pole, 

 until the last remnant is found in a few cells lying beneath the 

 ectoderm, forming a linear streak from the posterior end of the 

 embryo to the lip of the closing blastopore.'"'^ 



We have in the egg of the Toad-fish a condition intermediate 

 between such a form as Ctenolabrus and the Pigeon. The eggs of 

 these three forms represent a series in which the differences in 



^^Loc. eit., p. 68. 

 "Loc. cit., p. 12. 



