STEUCTUEE AT CLOSE OF EMBRYONIC PEEIOD. 73 



round the end of the notochord, and still communicates, though 

 by a very minute aperture, with the hinder end of the intestine. 

 Sense organs are represented by pigment spots in the wall of 

 the neural tube ; and a pair of small filaments, formed of 

 elongated and adherent cilia, and situated on the under sur- 

 face of the body behind the mouth, are very possibly taste 

 organs. 



One of the most interesting points to notice is that, up to 

 this stage, all the various parts of the body, the epidermis, the 

 walls of the neural tube and of the alimentary canal, the myo- 

 tomes, &c., all alike consist of single layers of cells, and cells 

 which, at any rate in their earlier stages of development, are 

 of epithelial origin. 



II. THE LARVAL PERIOD. 



This extends from the formation of the mouth to the critical 

 stage, at which latter date the mouth assumes its median position, 

 and the gill-slits become symmetrically arranged on the two sides 

 of the pharynx. The duration of the period is about three months. 



During the larval period, development proceeds far more 

 slowly than in the earlier stages. An interval of about a fort- 

 night is said to elapse between the formation of the first and 

 the second gill-slits ; and the close of the larval period, which 

 indicates a very definite stage in development, is also marked 

 by a pause of considerable duration. The chief events that 

 occur during the larval period are the formation of the gill- 

 slits of both sides of the pharynx, the formation of the endo- 

 style, the development of the atrial cavity, the shifting of the 

 mouth to its adult position, the establishment of the full number 

 of myotomes, together with certain important changes in their 

 relations to other organs, and the disappearance of the club- 

 shaped gland. 



Until recently our acquaintance with these stages was very 

 fragmentary, and due entirely to Kowalevsky's careful, but brief 

 and incomplete descriptions. Now, owing to Hatschek's observa- 

 tions on the development of the myotomes, and those of Willey 

 and Lankester on the formation of the gill-slits, atrial cavity, and 

 endostyle, we have far more complete and satisfactory knowledge 

 of the actual course of events, although there are many points 

 that still require investigation. 



