
1891.] Embryology. 1027 
Hertwig. ‘‘ Now the growth of the blastoderm over the yolk does not 
take place, according to Kupffer, in a true epibolic fashion, but is 
accomplished through the medium of a zone of tissue in which the 
yolk-cells become transformed into the cells of the two primary layers. 
Hertwig holds the same opinion. If this be really the case in Amniota, 
two explanations of the process are possible: First, that it is a modi- 
fication of the ancestral, epibolic growth, such as occurs in Teleosts, 
which view Kupffer and Hertwig would of course reject, because it is 
equivalent to admitting the homology of the blastoderm edge in 
Teleosts and Amniotes, and consequently the correctness of the Balfour- 
Rauber hypothesis ; secondly, that the process is, to refer it to simple 
embryonic forms, one of progressive delamination. It will be seen 
that Kupffer’s hypothesis really implies the occurrence of the latter 
process, for when he explains the spreading of the blastoderm as the 
completion of the blastula stage he really means that the embryo splits. 
off ectoderm progressively from a 3 a' towards m (anterior embryonic 
portion). Thus, again, to refer the processes to their simplest forms, 
over one half of the blastula yd half) delamination occurs ; 
but in the other half there is a true invagination (region of prostomia 
and primitive streak). . . . However, the analysis I have given is, I 
think, a perfectly fair one, and the result is evidently prejudicial to 
their theory. For the conclusion is that the Amniotic vertebrates have 
a blastula which invaginates over one half and delaminates over the 
other, Such an embryonic form is nowhere known to occur, and the 
theory which is forced to assume its existence is in so far a weak theory, 
and must give place to any other a can explain the facts by 
making use only of known processes.” Unfortunately for the author, 
the weak theory is probably to be regarded as a real fact, and if so 
‘the author’s argument collapses. Whether -or not a readjustment of 
the author’s views may show him to be largely in the right, is another 
question for the future to settle. Meanwhile any conception of fish 
‘(and Amniote) gastrulation must take into account as a starting point 
the delamination over the anterior and ventral face of the early stages 
of the frog’s development.—T. H. M., September gth, 1891. 
The Development of Hydra.2—The ground already covered 
by Kleinenberg (’ 72), Kerscher (’89), and Korotneff (83) has again 
been worked over by Dr. August Brauer, The following species of 
brown (‘not green’’) hydras were studied: Æ. grisea, Eggs and 
* Zeit. Wissen. Zool., Bd. 51, Heft. 2, 1891. - 



