705 



seems to me that a mass of tissue grows immediately backwards in 

 the middle line as the fish embryo elongates. At the same time the 

 material from the sides presses in towards the axial line to help in 

 the elongation. In addition material from the germ-ring continually 

 passes into the embryo, but not in sufficient quantity to form the sides 

 of the embryo. 



The accompanying diagrams for the fish and frog show in a rough 

 way by means of the arrows the differences in the method of formation 

 of the embryos. 



I find myself, therefore, in the anomalous position of denying a strict 

 process of concrescence (in His sense) in the classical fish-egg, and advocat- 

 ing something like concrescence for the frog. When, as pointed out 

 above, we recognise the different position of the egg occupied by the 

 fish and frog, it may not seem quite so remarkable to find the em- 

 bryonic material laid down in different portions of the egg, and the 

 formation of the embryos correspondingly different. 



Bryn Mawr College, 



Bryn Mawr, Pennsylvania, U. S. A. 



May 8th 1894. 



References to Literature. 



1) Hertwig, 0., Archiv f. mikrosk. Anatomie, Bd. 39, 1892. 



2) His, W., Unsere Körperform, 1874. 



3) His, W., Zeit. f. Anat. u. Entwickl. II, 1875. 



4) Joedan, Journal of Morphology, VIII, 1893. 



5) Morgan & Tsuda, Quart. Journ. Microsc. Science, Vol. 35, 1894. 



6) Morgan, T. H., Anat. Anzeiger, 1893. 



7) Pflüger, Archiv f. d. gesamte Physiologie, 1883. 

 •8) Roux, Anat. Anzeiger, 1888. 



