53 
moment the “critical stage” (WILLEY, 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 WYHE for 
the number of segments representing the cranial region 
in Amphioxus. 
First pair of Somites. — Opinions differ very much asto 
which segments and which gill-slits are to be regarded as 
the first pair. In a stage like that represented in fig. 5 
(p 211) 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. 
prost. The right one enlarges forwards 
and downwards so as to give rise 
brj _ to the head cavity ofthelarva. The 
left one remains much smaller, 
(0 
aes 
0e 
So) 
45 
ried 
z VE 
DAS re it acquires an opening to the 
ok zi Bike 7 exterior and is known henceforth 
SHE EED as the groove of HATSCHEK or 
ER Ene joe praeoral pit ') which after the 
Bee End formation of the tn involution 
ole MRE 7 opens into the lat 
| a Now jm (i892, p peld) 
considered both these “anterior 
ie grand slan Bn intestinal diverticula” as repre- 
fo) t f Amphio 
kn elden ee senting the first pair of gill-slits 
and showing tend and the two “rostral prolonga- 
ed oe first pair of gill- tions” as the first pair of somites, 
ke uches ( anterior ento- lying in front of them (l.c. p. 
AA rschex. 1882, fi 136-137) 
') The rightness of this statement, made eng by armed 
(1881, p. 73) and ae by MACBRIDE (1900, 
doubted by HATSCHEK (1906, p. 5) in a later article, eniassny unedr 
vp: 
the influence of theoretical Breaktuantignn. 
