AMPHIOXUS AND AMMOCCETES. l6/ 



Endostyle or Hypobrancliial Groove. 



At a stage in the development of Ammocoetes which 

 precedes the flattening out of the anterior gill-pouches, 

 a median depression occurs in the extreme anterior 

 region of the ventral wall of the pharynx between the 

 first pair of gill-pouches. In its production the wall of 

 the pharynx at this region projects itself ventrally and 

 slightly forward. This groove, which is known as the 

 hypobmnchial groove, develops in the direction from before 

 backwards, and eventually extends backwards as a longi- 

 tudinal groove as far as the fifth pair of gill-pouches 

 (Fig. 91). 



WILHELM MULLER showed that it was the homologue 

 of the endostyle of Ascidians and Amphioxus, and he has 

 been amply confirmed by DOHRN. It agrees with the lat- 

 ter structure in its origin at the anterior extremity of the 

 pharynx and subsequent growth backwards and in its 

 histological structure, the most marked feature of the lat- 

 ter being the four longitudinal rows of gland-cells which 

 were noted above in the endostyle of Amphioxus. (Cf. Fig. 

 13.) Like the latter, also, it is a slime-secreting gland. 



In Ammocoetes the hypobranchial groove becomes 

 largely shut off from the cavity of the pharynx by the 

 gradual ingrowth of a diaphragm-like lamella which pro- 

 ceeds from the angle made by the groove in front with the 

 anterior wall of the pharynx (Fig. 91). Subsequently a 

 similar diaphragm grows in from the posterior margin of 

 the groove, and finally the latter only communicates with 

 the pharynx by a small aperture in the mid-ventral line 

 between the fourth pair of gill-pouches. 



