54 



ZOOLOGY 



SECT, 



into a pair of pouches, which become shut off as closed sacs : of 

 these the right gives rise to the ccelome of the head (A), the left 

 to a depression called the prce-oral pit (w), from which the groove 

 of Hatschek is afterwards formed. On the floor of the archenteron, 

 in the neighbourhood of the mouth, a depression appears, which 

 gives rise to a structure called the club-shaped gland (k). 

 Posteriorly the neurenteric canal closes and the anus appears. 



We left the mesoderm in the form of separate paired ccelomic 

 sacs, arranged metamerically in the dorsal region of the embryo. 

 The sacs increase in size, and extend both upwards and downwards, 

 each presenting a somatic layer (Fig. 706, D, ink 1 ) in contact with 

 the external ectoderm, and a splanchnic layer (mk 2 ) in contact 



ch 



.c 



CIi 



J77T 



Ih ch 



Jr 



Fio. 70S. AmphiOKUS lance Olatus. A. young larva; B, anterior end more highly 

 magnified, c, provisional tail-fin; ch, notochord ; en, neurenteric canal; <t, enteric canal; 

 h, ccelome of head ; L\ club-shaped gland ; //, its external aperture ; A-*, first gill-slit : m. mouth ; 

 me. nerve-tube ; np.' neuropore ; s>:. sub-intestinal vein ; v:, prse-oral pit. (From Korschelt 

 and Heider, after Hatschek.) 



with the nervous system and notochord dorsally, and with the 

 enteric canal ventrally. At about the level of the ventral surface 

 of the notochord, a horizontal partition is formed in each ccelomic 

 sac (Fig. 706, D), separating it into a dorsal and ventral portion. 

 The dorsal section is distinguished as the protovertebra (//>s), and 

 its cavity as the myoccele or muscle-cavity : the ventral section 

 is called the lateral plate, and its cavity forms a segment of the 

 coelome. 



The ventral plates now unite with one another in pairs below 

 the enteric canal, their cavities becoming continuous : at the 

 same time the cavities of successive ventral plates are placed 

 in communication with one another by the absorption of their 



