214 WILLIAM BvVTESON. 



animal passes a prolonged period in this condition without any 

 external modification, but this, of course, is quite uncertain. 

 The larvte were all found between July 20th and September 

 6th, and I am in hopes that at some other season of the year 

 it may be possible to find the remaining stages between H and 

 the adult condition. 



Very considerable structural modifications must, of course, 

 occur before such a larva as Stage H can assume the external 

 form of the parent. These changes must consist chiefly in 

 a great increase in the length of the body, iii the number of 

 the gill slits, &e. ; but probably the point of greatest interest 

 lies in the subsequent development of the collar, which in the 

 adult presents an appearance considerably diflerent from that 

 of the same structure in this larva, both in position and extent. 



Internal Structure. — Having dealt with the external 

 appearances, I will now describe in detail the internal struc- 

 ture from the blastosphaere to Stage E. 



An account of the formation of the mouth and nerve-cord I 

 also introduced into this paper, but I propose to leave the 

 description of the other changes in Stages F, G, and H, together 

 with an account of some points in the anatomy of the adult, 

 to be given in a subsequent paper which I hope to prepare 

 shortly. 



The characters of theblastosphaere are shown in fig. 8. 

 The walls are formed of a layer of single cells enclosing a 

 segmentative cavity. This individual has already become 

 slightly flattened, having previously been spherical as shown in 

 fig. 4. Simultaneously with this compression a difi'erentiation 

 has commenced between the cells which are to form the epi- 

 blast {E) and those which will be invaginated to constitute the 

 hypoblast {H). The former are smaller and less granular than 

 the latter, which are large and contain many yolk particles in 

 their peripheral ends. The central ends of the hypoblastic cells 

 have a characteristic amoeboid appearance ; between these two 

 parts of the cell the large dotted nucleus is placed. 



Between the epiblastic and hypoblastic portions the walls are 

 composed of iudifi'erent cells. 



