128 



DEVELOPMENT OF AMPHIOXUS. 



rad- 



-lacf 



and constitutes the cavity of the snout lying below the 

 notochord (Fig. 63 C). 



Shortly after the breaking through of the mouth the 

 left sac acquires an opening to the exterior on the left side 

 of the body (Fig. 64). The right sac becomes the praoral 

 body-cavity or coslom of the "head," while the left sac is 

 known as the pya;oral pit. It is necessary to emphasise 

 the fact that these two structures which are so different 



in their fully formed con- 

 dition are at iirst perfectly 

 similar and symmetrical and 

 form a pair of "head-cavi- 

 ties." Ultimately, as we 

 have seen, only one of them 

 actually persists as a head- 

 Fig. 71. —Anterior portion of em- cavity ; namely, the right One. 



bryo, with thirteen primitive somites, -j-j^^ ^^^^^^ conversion of 



from the ventral side m optical section. 



(After hatschek.) the left sac into the prasoral 



r.a.d. and l.a.d. Right and left head- . . 



cavities, cs.g. Rudiment of club-shaped pit IS probably to be regarded 

 ^'^"■^^ as a secondary or cenoge- 



netic phenomenon, but the acquirement of an opening to 

 the exterior is probably not secondary, since a similar 

 opening (the proboscis-pore) occurs in Balanoglossus. 



In addition to the above-described peculiarities which 

 sufficiently distinguish the head-cavities from the myoco;- 

 lomic pouches, must be mentioned the fact that at no point 

 of their epithelial walls are muscles developed. 



It is probable that the pr^oral head-cavities of Amphi- 

 oxus are homologous with the prcEvtaudibular cavities of 

 the higher Vertebrates, from the walls of which the greater 

 number of the eye-muscles are developed.* This view is 



♦ This is also the opinion of Kupffer. Singularly enough van Wijhe has 

 advanced the view that only the right head-cavity of Amphioxus is to be 



