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spacious than that formed under the ventral lip, known as 
the anal gut. The yolk-laden central entoderm cells, however, 
evidently play a very passive role; no doubt they contribute 
by continued divisions to the production of the smaller 
and more active entodermcells surrounding them, but for 
the rest they are sunk into the interior only by the action 
of these peripheral cells and, until the last moment before 
the closure of the blastopore, remain visible in the opening 
as the “yolk-plug.” They are surrounded by the ring-shaped 
archenteron-incision which, as stated, is much deeper and 
wider anteriorly than under the posterior lip and is lined 
by smalier and more“active entoderm-cells. 
eory of concrescence.— The peculiar mode of con- 
traction of the blastopore border has given rise to diver- 
gent opinions. In the first place the concrescence-theory 
must be mentioned here. It was founded by HIS (1876) 
who was led to it especially by the study of the develop- 
ment of Teleosteans. According to him, the formative 
material for the embryo is situated originally as a ring 
round the border of the blastopore. The closure of the 
blastopore is performed by the concrescence of the lateral 
borders from the left and the right. This process occurs 
at the anteriormost point of the blastopore-border and 
the fusion proceeds from in front backwards. The two 
halves of the ring-shaped embryonal rudiment in this 
way unite in the middle of the anterior border of the 
blastopore and the embryo is formed in front of this point 
at the same rate as this moves backwards. This concep- 
tion was extended to other Chordates, from Amphioxus 
onwards, where HATSCHEK (1881, p. 31, 32) assumed 
concrescence though recognizing that it can not be observed. 
phylogenetic interpretation has been given to this 
process by HUBRECHT (1902, p. 69) in the following way. 
In his well-known theory on the origin of metamerism 
SEDGWICK (1884) derives the Annelids from an Actinia-like 
ancestor in which the opposite borders of the mouth-slit 
would have coalesced, leaving an anterior and a posterior 
opening, the mouth and the anus of the worm, while the 
diverticula of the gut which are separated by the septa 
pass into the mesoderm segments. This same principle is 
applied now by LAMEERE (1891) and afterwards by 
HUBRECHT to the Vertebrates which by these authors are 
derived in the same way from an elongated Actinia. LAMEERE 
