CTENOPHORA. 19 



and towards the epithelium outside the depression, which has Hkewise the nuclei arranged rather 

 regular!}' in a single layer. — The section described of the apical organ of this stage upon the whole 

 corresponds very well to that through the apical organ of Callianira bialaia represented by Her twig 

 (Op. cit. Taf. V, Fig. 8), showing that the organ in the earliest stages has exactly the same structure 

 as in typical Ctenophores. — There is yet no indication of the excretory canals and pores. 



The costs are formed as simple thickenings of the ectoderm, slightly depressed; in the ca- 

 vity thus formed between the epidermis and the egg-membrane lies the comb (PI. IX, Figs, i — 3. 

 PI. VIII, F'ig. 6). The nuclei are somewhat larger than those of the epidermis; they are arranged 

 more or less regularly in a single layer near the basis of the cells. It is an important fact that each 

 comb is formed from the first beginning by a considerable number of cells, not by a single cell as 

 was found by Chun (Monogr. p. ni, Taf. XVI, Fig. 22) to be the case in Eucharis multicornis. 



The tentacles are seen to originate as thickenings of the ectoderm; in horizontal sections 

 they have the .shape of a T (PI. IX, Figs. 2— 3. tb.); the thicker part in the middle represents the ten- 

 tacle itself, from the wing-like expansions originates the thick coUoblast-forming layer covering the 

 sides of the tentacle-base. In PI. VIII, Figs. 10 — 11 are represented two vertical sections through the 

 tentacle rudiment of this stage, fig. 10 of the lateral, fig. 11 of the thick, median part. The develop- 

 ment of the tentacle muscles has not yet begun. It would appear to be evident from these sections 

 that the tentacle apparatus originates alone from the ectoderm as maintained by Chun (Monogr. p. 14) 

 and Hertwig (Op. cit. p. 45), and quite recently by Hatschek (Das neue zoologische System. 1911; 

 p. 9). There is no trace of either mesodermal or entodermal elements taking part in its formation'). 

 — As seen in the two figures quoted the tentacle sheath has begun to form as a simple folding of 

 the ectoderm over the tentacle rudiment. 



Of the mesoderm in this stage I can only give the negative information that no undoubted 

 mesoderm cells could be observed in the sections and that no muscles have been formed as yet. 



It should not be omitted to state that the epidermis in the sections of these stages is not in so 

 complete condition as is shown in the figures, being broken in several places, but, of course, the restor- 

 ations are made onh' after careful studies, being in full correspondence with reality. It may also be 

 said here that in the sections of this and the following stage all wrinkles of the epidermis, which 

 are' evidently quite accidental results of the preservation, have been omitted. 



Stage III. In this stage (PI. II, Figs. 8— 11, PI. Ill, Figs.i— 2, 4, 6, PI. VIII, Figs. 12-14, PI. IX, 

 Figs. 6 — 14, PI. X, Figs. 1 — 9), by far the most important of the embryonal stages, the embryo is a fully 

 formed Cydippid with all the structural features characteristic of this form typically developed. Though 

 the\' are still enclosed within the egg-membrane, they are upon the whole rather well preserved (— the 

 membrane lies more loose and is ruptured in some of the specimens — ), so that it has been possible 

 to study their anatomy rather fully, by sections as well as by direct preparation under the microscope. 

 I may thus give a detailed account of this stage, which gives the clue to the anatomy of the grown 

 animal and also gives valuable hints as to its relations to other Ctenophores. 



■y Conip. E. Metschuikoff. Vergleichend enibryologische Studien. 4. Uber die Gastrulation und Mesodermbilduns; 

 der Cteuophoren. Zeitschr. f. wiss. Zool. 42. 1885. p. 653. P. Saniassa. Zur Histologic der Ctenophoren. Arch. f. mikrosk. 

 Anat. Bd. 14. 1892. p. 1S9, 192 — 193. K. Camillo Schneider. Lelirbuch d. vergleichendeii Histologic d. Tiere. 1902. p. 1S5. 



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