248 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
iu deriving the Wolffiaa duct from the coelom. What Brook mistook for the duct was 
very iirobably the lateral line anlage. . - 
While the anlage remains an undivided sac the surface view, Fig. 149, and the ! 
section, Fig. 105, fairly represent its character. During its existence as a simple sac ! 
it increases in length considerably and at the same time travels from its early posi- 1 
tion in front of the somites to a position some little distance behind the first or second f 
somite (compare Figs. 149 and 150). As to the manner in which the sac travels back- | ; 
ward, I can only say that it does not plow its way through the surrounding jiassive ^ | 
ectoderm, as Beard (5) states is the case with the lateral line anlage of Selachians. ^ ,! 
Its continuity with the surrounding ectoderm prohibits this idea. ) ^ 
The formation of the separate sense organs begins some 10 or 15 hours before j | 
hatching. The elongated sac suffers a constriction and gradually becomes divided ; 
into two parts, which are at first connected by a strand of cells. During the constric- j 
tion the backward growth of the whole anlage continues, so that the final position of 
the first (anterior) lateral line organ is some distance behind the first somite (Fig. 
150, Z. Z. 0 ^). Of the two parts into which the sac is divided, the anterior is the smaller 
and becomes the first (anterior) lateral line organ. The posterior portion by continued 
division gives rise to the remaining organs. 
By the time the sac is divided into two distinct parts (Fig. 150, right side) a gen- 
eral change in the character of these parts, as contrasted with the early condition of 
the organ, is perceptible. The change especially affects the anterior part, as may be 
seen on comparing Fig. 113, PI. ci, with Fig. 105. Fig. 113 is through the ante- j 
rior half of the parent sac, which is connected with the posterior half by a short strand : 
of cells. In the figure it is seen that the wide mouth of the jiareut sac no longer 
exists, and the connection of the sense-organ cells with the surface ectoderm is also ’ i 
of a different character The sense organ appears to have been constricted off from ; | 
the general ectoderm, and in consequence the nervous layer as well as the ei)iderniic 
stratum now passes over the mouth of the cavity. The cavity has also changed its , | 
character. It is much shallower and (compare Fig. 150, Z. Z. ok) is indeed almost •, 
spherical. It continues to grow shallower (Fig. 120, PI. cii, Z. Z. ok) until it finally J 
disappears (Fig. 127, PI. era) or perhaps is represented by the slight superficial | 
concavity which the sense organs of the lateral line possess at the time of hatching. 
During the later stages of its existence the cavity is so sharply outlined (Pig. 120, 
PL cii) as almost to suggest the presence of a cuticle. I have no stages in the de- 
velopment of the sense organ between that shown in Fig. 120 and the condition iu ; 
Fig 127, but the path followed is probably the same as that pursued by the branchial i 
sense organ : as the cavity of the organ continues to flatten out, the surface ectoderm 
dips into and lines it ; only in this case the nervous layer must be pressed aside, for iu 
the perfect organ (Fig. 127, PI. ci) the sense cells are covered by the epidermic stra- 
tum alone. The histology of the sense organs of the lateral line is quite like that of 
the branchial sense organ already described. : \ 
The constriction by which the first lateral line organ is separated from the surface 
ectoderm affects all the rest of the original anlage. Both the connecting strand of « 
sense cells and the posterior portion of the original sac are separated from the epider- I 
mic stratum by the nervous layer. Figs. 122 and 121, PI. cii, are sections through 
different stages in the formation of the connecting strand {con. st.). In Fig. 122 the ^ 
arrangement of the cells indicates the former presence of a cavity and suggests its * 
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