EARL! LARVAL DEVELOPMENT. 549 



essentially agrees with that of the Vertebrates, consisting- of vesicular 

 flattened cells. The structures thai were described as choixla-plates 

 appear to be a kind of artifact. 



Great changes also take place in the entoderm. Two lateral 

 diverticula first become abstricted from that anterior part of the 

 entoderm-sac which lies in front of the first primitive segment; these 

 diverticula lie beneath the cephalic process of the first primitive 

 segment (Fig. 286, do), and arc called by Hatschek the anterior 

 entoderm-sacs. This author has recently claimed them as an anterior 

 pair of branchial sacs (No. 8). The future fate of these two diverti- 

 cula, which at first resemble each other, differs greatly. The one on 

 the right (Fig. "JS7, </>') increases considerably in size and, pressing 

 back the anterior end of the alimentary canal, completely Alls the 

 anterior cavity beneath the chorda in the snout-like prolongation of 

 the body (Fig. 288, h). The left diverticulum (Fig. 287, dv), on the 

 contrary, remains small and does not shift forward. In later stagas 

 it lies transversely beneath the chorda and opens outward in front of 

 and above the oral aperture (Fig. 288, w). A strongly ciliated efferent 

 portion can be distinguished from a smaller blind portion lying to the 

 right. This vesicle to which, later, a nerve runs, was regarded by 

 Kowalevsky as a peculiar sensory organ of the larva. The pre-oral 

 pit becomes enclosed by the developing oral hood and thus comes to 

 lie within the adult mouth ; the ciliated epithelium lining its efferent 

 section increases in extent and gives rise to the so-called wheel-organ 

 (Raderorgan), while its sensory organ persists in the adult as the 

 groove of Hatschek. 



The two anterior entoderm-diverticula have been interpreted in many 

 different ways. We have already mentioned that Eatsi her (Xo. 8) regards 

 them as the most anterior pair of branchial sacs. van YVijhe (No. 22) 

 recently claimed the aperture of the left entoderm- vesicle as the primary 

 mouth (autostomai of Amphioxus, homologous with the mouth of the Tunicata. 

 The ciliated organ which develops from the entoderm-vesicle, together with 

 Hatschek's pit, has been compared by van Wijhk to the ciliated pit of the 

 Tunicata. The right entoderm-vesicle, on the contrary, which is not morpho- 

 logically the antimere of the left, corresponds to the anterior cephalic somite of 

 i he Selachians, from which are developed the optic muscles innervated by the 

 oculo motorius. Batesoe (Xo. 26) has compared the two anterior vesicles to 

 the proboscis-coelom of Balanoglossus and the aperture of the left vesicle to 

 the proboscis-pore, a conclusion with which Wiu.kv (No. XIII.) agrees. 



A further derivative of the entoderm is found in the so-called club- 

 shaped gland, first seen by M. SCHULTZE and later by LEUCKART and 

 PAGENSTECHEB I Figs. 288, 289, If). This lies near the first primitive 



