CTENOPHORA. 



17 



on fresh material are necessary for solving this question also. — The arrangement of the brood-cavities 

 is quite irregular (Comp. PI. I, Fig. 9). They are confined to the sides of the body, in larger specimens 

 also occupying part of the chinniey-wall in the lower part; higher up on the sides of the chimneys 

 than the level of the genital organs I have not found them (comp. PI. IV, Fig.s. 2 — 4; e. c). In larger 

 specimens the}- may be so crowded as to lie in a dotible layer; I have counted up to 35 in one speci- 

 men. The embr\os occur in all different stages of development, arranged without an\- order what- 

 ever, in the same specimen. 



The first stages of the development, cleavage etc., I have not been able to^work out — not 

 because such younger stages were not found, but because the thick egg-membrane has prevented the 

 penetration of the preserving fluid, the young cells being thus not satisfactorily fixed. I think I 

 have observed the large, clear entodermal cells typical for Ctenophoran development; but upon the 

 whole these young stages were not sufficiently well preserved for being worth figuring. 



Stage I. The youngest embryos sufficiently well preserved for showing anything clearly are 

 those represented in PI. II, Figs, i — 3 and 7, this stage being designated as stage I. The shape is 

 quite spherical, the diameter being ca. i-2"'"'. At the apical pole of the embryo is seen an epithelial 

 thickening, elongated in the sagittal plane (PI. II, Fig. 3, .s.). It is the apical organ, the two 

 elongations representing the polar fields. The shape is quite similar to that shown in Taf. VII, 

 Fig. 20 of Chun's Monograph, representing a young Be roc' Forsknlii. The costa; have made their 

 appearance as four pairs of radiating, thickened lines, in which there is already a distinct grouping 

 of the cells in transverse series; from each of these groups a comb originates, though not yet seen in 

 this stage. As seen in side view (PI. II, Fig. 2) the costse reach nearly halfway down on the embryo. 

 At the level of the lower end of the costfe is seen a pair of epithelial thickenings with a vertical 

 keel in the middle; these represent the rudiments of the tentacle apparatus. On the oral side 

 there is a distinct furrow in the transverse plane, reaching scarceh' halfway up to the tentacle-rudi- 

 ments (PI. II, Figs. I — 2). The mouth-opening is not seen, the edges of the transverse furrow lying 

 close together. In the slightly younger stage represented in Fig. 7, PI. II, the mouth is very distinct, 

 the transverse furrow having as yet scarcely begun to form. (In this stage the aboral side is as in 

 figs. 2 — 3, the costge being only a little less developed). The four radiating lines seen in this figure I 

 suppose to be due to the arrangement of the large entodermal cells; it cannot, however, be decided, 

 the sections giving no information thereof on account of the bad preservation of the entodermal ele- 

 ments. I would refer to such figures as Taf. VII, Fig. 11 and 14, Taf. VIII, Fig. 9 — 10 of Chun's 

 Monograph as affording the probable explanation of these radiating lines. Whether the radiating 

 darker tracks seen in Fig. 6, PI. II should be explained in the same wa\- or perhaps represent the 

 mesoderm-stripes I do not venture to maintain; the sagittal tracks might be due to the pharvnx, which 

 most probably is the explanation of llie vertical (sagittal) tracks seen in Figs, i and 4, PL II. 



The embr>os in this and the other stages .show a number of small, irregularly placed, white 

 spots. I suppose they are only effects of preservation (coagulated matter), but as I cannot give 

 definite proof thereof, I have thought it right to mention the fact and represent it in the figures of 

 this stage. 



The egg-membrane lies quite close to the surface of the embryo; only over the transverse 



The IngoU" Expedition. V. 2. ■^ 



