Bd. VI: 4) THE ECHINOIDEA. 75 



SIZ (»Challenger» Echinoidea, p. 177. PI. XX. a.) and recently (under the name of 

 Abatus cavernosus) in the »Panamic Deep Sea Echini* (p. 211. PI. 99), and also 

 LOVEN has given some excellent figures of a young specimen of the same species 

 (named Abatus cavernosus) in his memoir »On Pourtalesia» PL XIV. The correspond- 

 ing stages of A. cavernosus agree very closely herewith, as might be expected on 

 account of the near relation between these two species. The following stages, show- 

 ing the development of the fasciole and the ambulacra, and, upon the whole, the 

 transformation of the test from the embryo to the fullgrown Echinoid were hitherto 

 unknown. I have thus thought it desirable to use this rare occasion for giving a 

 full account of the changes undergone during growth by this species. — Upon the 

 whole the development of A. cavernosus corresponds rather closely with that of 

 Brisaster fragilis (comp. »Ingolf» Echinoidea. II. p. ill — 114. PL XIII), a fact of 

 considerable importance for judging of the relationship of the genus Abatus. 



It may first be stated that in one and the same marsupium may be found newly 

 laid eggs, embryos with the spines just appearing and fully developed young ones, 

 ready to leave the marsupium. 



The first developmental stages: cleavage etc. I am unable to work out satis- 

 factorily on the material before me; for this purpose a large material preserved 

 specially for such research will be needed. Evidently the early development of this 

 species will prove to be very interesting. On account of the large size of the eggs 

 (a little over 1 mm. in diameter), there must necessarily be considerable differences 

 from what obtains in Echinoids with small eggs and pelagic larva;. As far as can 

 be ascertained on the material in hand, the cleavage proceeds in the same way as 

 that described by me for Cucumaria glacialis LjG., the nuclei lying scattered in 

 the yolk and later on wandering towards the surface of the egg, the great yolk- 

 mass remaining undivided. * 



Regarding the development of the test, the first stage seen is that figured in 

 PI. XVIII. Figs. 3 — 4. The embryo at this stage measures 1.5 mm in length. It is 

 seen that the development of the actinal skeletal plates precedes that of the abactinal. 

 From the actinal side the calcareous plates spread out and, as it were, overgrow the 

 yolk. The embryo in this stage is generally more or less irregular in shape on 

 account of the mutual pressure in the marsupium; the actinal side, and so much of 

 the abactinal side as is already covered by the coronal plates, has assumed the regular 

 shape; but the apical skeleton still consists only of small isolated plates, which can- 

 not overcome the irregular shape of the embryo. Contemporarily with the development 

 of the apical system into a compact covering the embryo acquires its regular shape. 



The explanation of the actinal skeleton (PL XVIII. Fig. 3) causes no difficulties. 

 The primary interambulacral plates are seen to project adorally beyond the ambu- 



* Zur Anatomie imd Entwicklung der Cucumaria glacialis. Zeitschr. f. wiss. Zool. LVII, 1894. p. 721. 



