126 bulletin: museum of comparative zoology. 



The ectoderoiic invasions occur in the same way as in the posterior zooid 

 and in corresponding positions, but the one which cuts off a lateral por- 

 tion of the dorsal muscle baud is ultimately repeated here for each new 

 segment, and occurs at the place where subsequently the dorsal bristle 

 bundles are found. In fact, these ingrowing elements constitute the 

 bristle forming organs. It is further to be noted that, at the time of 

 separation of the zpoids, the bristle bundles of the anterior worm are in 

 a much less advanced state of development than those of the posterior 

 worm. Another point of difference is found in the fact that there is a 

 much more intimate fusion of the several ectodermic ingrowths of the 

 ventral and lateral regions with each other and with mesodermic ele- 

 ments in the anterior than in the posterior zooid. In this manner is 

 produced an indifferent zone, similar in all particulars to the undifferen- 

 tiated segment-forming zone previously mentioned as characteristic of 

 the preanal region. From the ectoderm comes, as in the posterior zooid, 

 the mass of tissue from which ventral nerve chain, ventral bristle sacs, 

 etc., are developed. All stages in the differentiation of these structures 

 may be found in a rapidly growing worm, as one passes forward. Im- 

 mediately in front of the anal segment, where the tissue is least differ- 

 entiated and the ventral nerve cord is to be developed almost de novo, 

 an ectodermic ingrowth in a mid-ventral position divides the ventral 

 longitudinal muscle and contributes to the ventral cells of the cord 

 (Plate 3, Fig. 15 ; Plate 4, Fig. 23). Proliferations which are lateral, 

 but still penetrate the ventral muscle, add cellular elements to the mar- 

 gins of the cord (Plate 2, Fig. 8; Plate 5, Fig. 24, gn.v.). Thus in the 

 formative region the cord can be resolved into a median and two lateral 

 constituents. It was the latter which Semper ('76) regarded as meso- 

 dermal. The fibres of the nerve oord naturally become less numerous 

 posteriorly. The more dorsal, i. e. the deeper fibres, are the first to ap- 

 pear ; this produces the condition figured in Plate 3, Fig. 15, in which 

 the fibrous tract appears to occupy a more and more dorsal position as 

 one proceeds toward the tail. The ectoderm on the ventral side of the 

 body in this region is especially thickened, being four or five layers of 

 cells deep (Figs. 15, 16, 22, 23, 24). 



^ h. Entoderm. 



It is to be borne in mind that before the be^innino: of division the di- 

 gestive tract in the region affected by that process is a simple tube with 

 interseptal enlargements. The wall of the intestine, reckoning from the 



