124 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



the formation of the mouth. The apparent depth of the ectodermio 

 depression is increased by a pair of growths (directed backward and out- 

 ward) of the thickened ectoderm anterior to the groove in the latero- 

 ventral regions, to form the digitiform appendages (Figs. 16, 17 

 pr^c. dg.). 



The process of ectodermic ingrowths between and through the muscle 

 bands, referred to above, is kept up both anterior and posterior to the 

 plane of separation of the zooids. 



In the posterior zooid there are five distinct regions of cellular activity 

 in the ventral part of the body, namely : (1) the ectoderm in the spaces 

 between the lateral and ventral bands of longitudinal muscles (Plate 1, 

 Figs. 6, 7 ; Plate 2, Figs. 8, 9) ; (2) the ectoderm invading the ventral 

 muscle band and cutting off certain fibres (Plate 1, Figs. 6, 7, mu. v. 

 and mu. v'.') at either margin ; and (3) the cellular elements of the ven- 

 tral nerve cord itself. More dorsally, in the spaces between the dorsal 

 and lateral longitudinal muscles, there is likewise an ingrowth of ectoderm 

 on either side of the body, which in some measure supplements and fuses 

 with the masses already described. More dorsal still, approximately at a 

 level with the dorsal wall of the gut, is seen (Fig. 7) another irruption of 

 ectoderm so located as to cut off a strand from the ventral margin of 

 the dorsal muscle. This ingrowth differs from the former ones in the 

 very important fact that its extent in an antero-posterior direction is 

 extremely limited. The former proliferations occur one after another in 

 longitudinal series, one of each series to every segment. The ingrowth 

 through the dorsal muscle band, on the contrary, is limited to a single 

 pair of cell masses situated well forward towai'd the plane of division. 

 The cells from this pair of ingrowths move dorsally and toward the 

 median plane until they rest upon the digestive tract, and finally meet 

 each other, forming the brain ganglia, whose elements later produce a 

 connective (Plate 1, Fig. 6, conH. crc'ce.). The detached portion of the 

 dorsal muscle band approximately equals in dimensions the whole lateral 

 band. The pair of irruptions through the marginal portions of the 

 ventral muscle, in conjunction with elements from the ventral nerve 

 cord itself, furnishes the material for the cord in the newly forming seg- 

 ments of the anterior zooid and the infra-pharyngeal enlargements of the 

 nerve cord in the posterior zooid. In the posterior worm there is a 

 degeneration of that part of the old nerve tract which lies in front of 

 the place to be occupied by the infra-pharyngeal ganglia, only a few- 

 fibres remaining to preserve connection between the individuals until 

 separation is accomplished. 



