92 ADAM SEDGWICK. 



in some forms of Vertebrata to the ingrowth of the mesoblast 

 from the lips of the blastopore. 



" It is therefore highly probable that the paired ingrowths 

 of the mesoblast from the lips of the blastopore may have been 

 in the first instance derived from a pair of archenteric diver- 

 ticula. This process of formation of the mesoblast is (as may 

 be seen by reference to the summary (pp. 291, 292), the most 

 frequent, including as it does the Chaetopoda, the Mollusca, 

 the Arthropoda," &c. {' Comp. Emb.,' vol. ii, pp. 293, 294). 



It has been supposed until quite recently that only one pair 

 of diverticula are developed (except in the Echinoderms and 

 Balanoglossus) . But Hatschek has shown that in Amphioxus, 

 a very primitive and isolated animal, a series of diverticula are 

 formed, each diverticulum giving rise to a mesoblastic somite, 

 or, to put it in another way, that the lateral walls of the 

 archenteron become folded before the region of the archenteron 

 which they limit become separate from the central part of the 

 archenteron. Amphioxus is the only segmented animal in 

 which the body cavity is known to arise directly from archen- 

 teric pouches ; development of the coelom in other segmented 

 animals being regarded as an abbreviation of a similar process. 

 Now, however, that we know that the body cavity of Amphioxus 

 is developed from a series of archenteric pouches, it seems to me 

 that we are justified in concluding on similar grounds that the 

 abbreviated development in other segmented forms is derived 

 from a similar process.^ 



So that the difference between a segmented and an unseg- 

 mented animal consists in this, that in the former the archen- 

 teric walls become more folded than in the latter and give rise 

 to a greater number of pouches, each of which becomes a 

 mesoblastic somite. This is exactly the difference between a 

 Hydra and a Medusa. 



The similarity between the diploblastic Amphioxus embryo 

 with a pouched gut (pouches giving rise to the mesoblastic 

 somites) and an Actinozooid polyp or medusa suggests 



1 This has been already pointed out by Hubrecht ; see Hubrecht, " On the 

 Ancestral Form of the Chordata," this Journal, 1883. 



