ORIGIN AND STRUCTURE OF THE HEAD 



119 



moment the "critical stage" (WlLLEY, 1891, p. 202) is reached, 

 the larva sinks to the bottom and gives up pelagic life. Thus 

 WiLLEY (1894, p. 174) concludes that the common ancestor 

 of Acrania and Craniates has had some 9-14 gill-slits, a 

 somewhat higher number than found by VAN WVHE for 

 the number of segments representing the cranial region 

 in Amphioxus. 



First pair of Somites. — Opinions differ very much as to 

 which segments and which gill-slits are to be regarded as 

 the first pair. In a stage like that represented in fig. 5 

 (p. 25) it seems fairly evident which pair of mesodermic 

 segments is the first. Afterwards, however, these two ante- 

 rior somites each send out a prolongation into the snout, 

 the "rostral prolongations' which also form muscles and 

 supply the mesoderm to the dorsal part of the snout, which 

 is situated over the notochord. At the same time the ento- 

 derm just in front of the first pair of mesodermic segments 

 produces two diverticula (Fig. 7 and 23, br. 1) which have 



an entirely different fate. Both be- 

 come detached from the entoderm. 

 The right one enlarges forwards 

 and downwards so as to give rise 

 to the head cavity of the larva. The 

 left one remains much smaller, 

 it acquires an opening to the 

 exterior and is known henceforth 

 as the groove of Hatschek or 

 praeoral pit ^) which after the 

 formation of the mouth involution 

 opens into the latter. 



Now HATSCHEK (1892, p. 144) 

 considered both these "anterior 

 intestinal diverticula" as repre- 

 senting the first pair of gill-slits 

 and the two "rostral prolonga- 

 tions" as the first pair of somites, 

 lying in front of them (1. c. p. 

 136-137). 



Fig. 23. Anterior end of a 

 young stage of Amphioxus, 

 seen from the ventral side 

 and showing the formation 

 of the first pair of gill- 

 pouches ("anterior ento- 

 derm pockets', br. 1), after 

 Hatschek, 1882, fig. 53. 



The rightness of this statement, made originally by himself 

 (1881, p. 73) and confirmed by MacBride (1900, p. 363), has been 

 doubted by Hatschek (1906, p. 5) in a later article, evideatiy under 

 the influence of theoretical considerations. 



