EMBRYOLOGY OF THE SEA BASS. 
26.5 
homologies are as follows : Yolk together with the periblast represents the yolk cells of 
the Amphibian gastrula ; the invagination at the posterior pole of the fish blastoderm 
represents the invagination round the dorsal lip of tlie Amphibian blastopore, which 
forms the so-called chorda- eutoblasr ; the gastrula cavity in the fish is morphologically 
between the invaginated layer and the ijeriblast. Ziegler’s theory is concisely stated 
in the following quotations (48) : 
Bei der Unite nud den Salinoniden wird die Bildung der nnteren Scliicbte am ganzen Eande der Keiin 
schoibe oingeleitot ; sie beginnt aber an der dorsalen Soite friiber als an der andercn, und scbreitotnnr 
anf dieser fort, wiibrend sie ini iibrigen Umfang liald wieder sistirt wird. 
Pbylogenetiscb ist das Entoderm der Teleostier ntir der dorsalo Tbeil des Darmdriisenblattcs ; es 
entsteiit aber aus deuiselbeu das ganze Darmepitbel, indem es niedianwiirts aufstiilpt, nnd daranf die 
so entstandene Einne vom Hotter abgescbniirt wird. 
Der bei diesom Vorgang in der Kiemengegeud entsteliende freio Eaum zwiscben Entoderm nndDotter 
entspricbt einem Tbeil der Gastrula nnd Darniboble der priniitiveren Entwicklungstypon ; dasselbe 
gilt vielleicbt von der Kupffer’scben Blase. 
Accepting Ziegler’s homologies, it will be seen that the whole course of the fish 
development becomes easy to understand. Starting with the blastula (Fig. 25, FI. 
xci, s, c., segmentation cavity) and disregarding for the present the non-embryonic 
part of the germ ring, the iirimitive hypoblast (p. li.) which invaginates at the poste- 
rior pole {p. p.) of the fish blastoderm (Fig. 9) corresponds to the primitive hypoblast 
{p. Ji.) which invaginates round the dorsal lip of the blastopore in the frog gastrula 
(Fig. 11). The chief point of difference is the lack of continuity in the fish embryo 
between the inner edge of the 
invaginated layer and the yolk, 
easily explained as an adapta- 
tion to the method of forming 
the alimentary canal from the 
invaginated layer exclusively. 
The archenteron {arch.) lies be- 
tween the itrimitive hypoblast 
{p. h.) and the i^eriblast (Fig. 9, 
|>. 2G3), In consequence of the 
absence of continuity between 
the yolk and the invaginate 
layer, the archenteron at its 
edge is not separated from the 
segmentation cavity [s.c.). The 
growth of the anterior ])ole of 
the blastoderm round the yolk 
(compare Figs. 9,p. 203, and 10, 
p. 204) represents the growth of 
the small cells round the yolk 
cells in Amphibian gastrula- 
tiou. The closure of the blasto- 
pore takes xilace in .the same 
way as in the Amphibia ; there is formed a short primitive streak behind the position 
of the neurenteric canal (Kupffer’s vesicle in Teleost) ; at the posterior end of the primi- 
tive streak the final closure takes i)lace (the comi^arisou must be made with those 
yrnes. 
y-c. 
Fifi. 11 (after nertwig, 20). — Median longitudinal section tlirougli frog’.'. 
ga,strnla — d. 1., dorsal lip of lilaato])oro; v. veiitr.alliTi of blastopore; d.ji., 
Dotterpropf ; s.c., remnant of segmentation cavity; y.c., yolk cells ; ec., 
octoderni ; p. /c, prim. hypobla.st ; arch., arclientoron : v. iiies., ventral nieso- 
blast. 
