CTENOPHORA. 



furrow it becomes free of the epidermis and thus can be seen in side view (from the transverse plane) 

 (PI. II, Fig. am.). 



Stage II. In the next stage (II), represented in PI. II, Figs. 4 6, the size is the same as in 

 the former stage, ca. rz mm in diameter, and the shape is also spherical as before. The combs have 

 been formed, though quite short as yet The costae are distinctly shorter than in the foregoing stage, 

 being more concentrated a very conspicuous feature; it should however be pointed out that the 

 number of the combs is rather variable, so that the difference in the size of the costae may be partly 

 due to this fact. Also the tentacle rudiments are somewhat nearer the apical pole than in the preceding 

 stage (comp. figs. 3 and 6, PL II). The transverse furrow is much deeper, but has still not nearly 

 reached the tentacle rudiment. As seen from PL III, fig. 8, which represents an embryo of this stage 

 opened, the furrow has already a considerable depth. The apical organ has been rather deeply sunk, 

 the body forming four rounded elevations round it. The egg-membrane still adheres closely to the 

 epidermis, except over the apical invagination, the transverse furrow and the costae (comp. PL II, 

 Fig. 5, PL III, Fig. 8m.; also PL IX, Figs. 1-5). 



On account of the thick, very resistant egg-membrane it is very difficult to have these yoxmg 

 stages imbedded and sectioned. Only by cutting a hole in the membrane have I been able to obtain 

 sections, which are somewhat satisfactory. The histological preservation is, as regards the entodermal 

 cells, very poor, so that no information can be given about the entoderm of the embryo; but the 

 ectoderm is tolerably preserved, the sections thus giving some information of interest. 



In PL IX, Figs, i 5 are represented some horizontal sections at different levels through an 

 embryo in the stage II; they are easily understood on being compared with PL II, Figs. 4 6 and 

 PL III, Fig. 8. In Fig. 2 the pharynx is cut on the level where it passes into the transverse furrow, 

 which latter in the following sections gradually lengthens until (Fig. 5) it reaches the periphery, thus 

 dividing the body (in the lower part) into two lobes. The walls of the furrow are rather thick, the 

 nuclei being arranged irregularly in two three layers. In PL VII, Fig. 4 is represented a longitudinal 

 section through the pharynx in this stage; it is seen to be distinctly ciliated. At the upper end 

 the walls continue into a thin layer of entoderm. A corresponding thin layer of entodermal cells is 

 seen in PL VII, Fig. 6 just below the apical organ; it represents the roof of the infundibulum. Farther 

 out both these entodermal membranes pass into the large, vacuolated entodermal cells, which are too 

 badly preserved for figuring. 



A vertical section through the apical organ of this stage is represented in PL VII, Fig. 6. 

 The epithelium has already been very considerably thickened. A group of otoliths (ot.) are seen lying 

 in the cavity of the depression apparently borne upon a bunch of cilia, which I think must be regarded 

 as homologous to the balancers of other Ctenophores. Other otoliths are also seen still lying between 

 the epithelial cells; they have each a rather distinct nucleus, in accordance with the observations of 

 Samassa (Op. cit. p. 181, Taf. IX, Figs. 16, 17). The cells within the depression carry a dense coat 

 of short cilia. At the edge of the depression there is a zone (c. z.) of high clear cells, with the nuclei 

 arranged rather regularly in a single layer near the outer surface, and which carry long, very distinct 

 cilia. They are the cells which form the cupule, the cilia being as yet isolated and not converging 

 over the otocyst. This zone is very distinctly limited both towards the epithelium of the depression 



