EMBRYOLOGY OF THE SEA BASS. 
269 
Hertwig’s explauation of the blastoderm edge is j)raetically the same as Kui)fler’s. 
Eabl (38) states his i)ositiou, which is identical with the preceding, by means of a 
neat comparison between the bird and the frog gastrnla, The line x-y, in Fig. 11, p. 205, 
cats the frog’s gastrnla in two parts. The part to the left corresponds to the Amniotic 
embryo, the part to the right to the Amniotic yolk. The blastopore {<1. l.-v. 1.) equals the 
primitive streak, all round which there is an ingrowth of cells. The part to the right 
of x-y becomes transformed entirely into yolk. Eabl does not offer an explanation 
of how the transition from one gastrnla to the other was accomplished. All he says 
is, that the effect is due to the great increase in size of the yolk (yolk cell mass). 
The inference, however, is that he adopts Cunningham’s explanation, and supposes 
an actual hernia to take place. 
The only argument for the view of the Amniotic gastrnla entertained by the above- 
mentioned authors is that all round the lip of the blastopore in the Amphibia, etc., 
thei’e is an ingrowth of cells, and that round the prostoma of rei)tiles and primitive 
streak of birds and mammals there is the same ingrowth. The two structures it is 
concluded must therefore be homologous. It then becomes a question of how to ex- 
plain the blastoderm edge. Two explanations of the structure have been offered, one 
by Kupffer which is substantially the same as that of Oscar Hertwig, and one by Cun- 
ningham to which Eabl seems inclined. Kupffer’s explauation is contained in the 
following inrssage (26) : 
Icli fasse also die Ausbreituug des Blastoderms liber deu Hotter als Blastulabilduiig aiif. Her 
Abscbluss dieser BilduDg ert'olgt um so spiiter je grosser der zii umwacliseiide Hotter ist, uud es tritt 
der Gastnilatiou vor Vollendung der Blastiilabilduug ein, d. wiilireud eiu Blastotroma [so-called 
Hotter-Blastoxiorus] uocli vorbaudeii ist. 
On analyzing Kupffer’s view it will beseen that by “ blastula ”he means not a one- 
layered but a two-layered embryo. 
His theory is illustrated by the dia- 
gram, Fig. 12, in which a-d mark the 
blastoderm edge of an Amniote ; s. c. 
is the segmentation cavity, and y is 
the yolk — the embryo being in what 
would commonly be called the blas- 
tula stage. Kow the growth of the 
blastoderm over the yolk does not 
take place, according to Kuj)fler, in 
a true epibolic fashion, but is accom- 
Tilished through the medium of a zone 
of tissue (Keimwall) in which the 
yolk cells (nuclei) become trans- 
formed into the cells of the two pri- 
mary layers. Hertwig holds the 
same opinion (Lehrb., p. 105). If 
this be really the case in Amniota, 
two explanations of process are 
possible: First, that it js a modifica- 
tion of the ancestral, epibolic growth 
(such as occurs in Teleosts), which 
view Kupffer and Hertwig would of 
R.str{prosiorm] 
m 
Fig. 12. — Diagram to illustrate KuplFer’s theory of tlie Amniotic gas- 
trula— s. c., segmentation cavity; y, j-ollc ; a. to a., eilge of lilasto- 
ileriii; P.str., primitive streak (Prostoraa of reptiles); del.l., line 
along wliicli, according to Kupffer, the yolk splits off ectoderm. 
