358 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
a cleft appears between it and tlie yolk. At this stage there are there- 
fore two cavities, the anterior segmentation-cavity, and the quite distinct 
posterior cavity formed as above, and opening at the blastopore. The 
two come together by a rupture of the separating partition, and an 
archenteron is formed. The roof of the archenteron usually gives clear 
evidence of its twofold origin in the different structure of its two sets 
of cells. In fact, the anterior and posterior regions of the roof do not 
lose their distinctness of character. The anterior (vegetative) layer 
grows backwards from the line of contact, insinuating itself beneath the 
posterior layer, and forming the definitive epithelium of the gut. 
Before the beginning of this undergrowth, the margin of inturning 
shows a posterior curvature of its edges, so that an anterior blastopore- 
lip and two lateral walls are formed. A posterior lip is formed after 
the complete incurving and coalescence of the two outermost ends of the 
margin of inturning. Bound the whole of the blastopore margin the 
ectoderm passes into the mesoderm ; the latter forms a unified rudiment, 
which (before the above-described undergrowth of the endoderm) con- 
stitutes part of the roof of the archenteron. From this region, as the 
endoderm grows under, the rudiments of the notochord and the lateral, 
plates are formed. At the same time the medullary plate, groove, and 
canal appear successively. The closure of the blastopore takes place 
from in front backwards ; by the closure of the anterior part the external 
communication of the neurenteric canal is shut off ; the internal com- 
munication with the archenteron remains open for a considerable time \ 
the most posterior part of the blastopore becomes the anus directly. 
Present Position of Darwinism.* — Herr G. Wolff has under this 
title published a lecture, which is almost wholly concerned with a criti- 
cism of Weismann’s £ Neue Gedanken zur Vererbungsfrage,’ an emphatic, 
compliment to the Freiburg professor, with whose views the lecturer 
finds himself much at variance. After emphasising the fact that Dar- 
winism postulates as data quantitative , rather than qualitative variations, 
Wolff seeks to show that Weismann’s doctrine of Panmixia is inconsis- 
tent with the Darwinian postulate, since Panmixia cannot be an effective 
factor except in regard to qualitative variations. It appears to us, how- 
ever, that a way out of this apparent dilemma has been already indi- 
cated by several writers on Panmixia, and by one at least — the late 
Mr. Bomanes. 
Herr Wolff proceeds to a criticism of the Panmixia idea, resting his 
argument very largely on the alleged fact that the ontogeny of dwindling 
structures points not to weakened development of the rudiments, as the 
doctrine of Panmixia might lead us to expect, but rather to a retrogres- 
sive metamorphosis. We must confess our inability to see any obvious 
difficulty ; and as to the case he selects — the occurrence of rudiments of 
teeth in modern birds— we think any authentic cases of this are far to 
seek. 
In support of Panmixia, Weismann has made use of the conclusion 
that unfavourable variations are more frequent than favourable variations ; 
but Wolff points out that there is no reason to believe that this is true 
of purely quantitative variations, such as Darwinism postulates. To 
avoid this difficulty Weismann has restricted the conclusion as to the 
* ‘ Der gegenwartige Stand des Darwinismus,’ Leipzig, 8vo, 1896, 30 pp^. 
