166 THE ANCESTRY OF VERTEBRATES 



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 smaller and more active entoderm-cells. 



Theory of concrescence. — The peculiar mode of con- 

 traction of the blastopore border has given rise to diver- 

 gent opinions. In the first pl^ce 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. 



A 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 

 diveiticula 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 



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