970 RECORD OF CURRENT RESEARCHES RELATING TO 



cells increase in size, their nuclei become converted into large germi- 

 nal vesicles. One of tliese vesicles continues to grow, and becomes 

 the definite germinal vesicle of the egg, while the other cells diminish, 

 and go to form part of the investing cell-layer. 



After describing the further details of the egg's growth, the author 

 points out that in the course of its development there appear two 

 directive corpuscles. The peripheral cells of the morula go to form 

 the blastoderm-cells ; the nuclei pass to the outer margin ; the cells 

 increase in length, and elongate in a radial direction. The endoderm 

 is formed by cells which arise in the central portion of the morula. 

 Two covering shells can be made out at about this stage. One is 

 formed from the parent, and one is shed out by the egg itself. The 

 latter, primitively simple, becomes thickened under the influence of 

 external adverse conditions. It was this shell-layer which Kleinenberg 

 regarded as the ectoderm of the embryonic Hydra. The internal 

 cavity is formed by the centrifugal growth of the cells of the embryo. 



Porifera. 



External Gemmation in the Spongida.* — M. de Mereschkowsky 

 directs attention to this mode of reproduction, which, as compared 

 with the other asexual method (that of the formation of gemmules), 

 has been but little studied ; the process indeed seems to be rare, and 

 lias hitherto only been observed in the four genera of the family of 

 the SuberitidinfB, Tethja, Tetilla, Suberites, and Rinalda. 



After referring to the observations of Bowerbank and Oscar 

 Schmidt, the author passes to the account which he has himself given of 

 Rinalda arctica (from the White Sea) ; here he observed that the whole 

 of the sponge was covered with long conical protuberances, hollow inter- 

 nally ; buds became detached from these cones, and fixing themselves, 

 gave rise to fresh sponges. 



The observations now. to be recorded w — ^ made on a species 

 of Tcthya from the same sea. This sponge is not more than one 



EXPLANATION OF PLATE XXL 



(Figs. 1-5 are natural size. Figs. 6-8 x 7, Figs. 9-11 x 40, Figs. 12, 13 x 10). 



Figs. 1-3. — Adult specimens of Te/hya from the White Sea. On the surface 

 are groups of buds ou peduncles of different form, length, and size. 



Fig. 4. — Probably a detached bud, throwing out in all directions long fila- 

 ments and protuberances, which later will bear buds at their extremities. 



Fig. 5. — Two specimens of tlie same Tethya attached to tlie interior surface of 

 a Tcrehratula, without buds or any kind of protubcrnuces on the surface. 



Fig. 6. — On a long peduncle originating from the mother-body x, is a bud of 

 irregular form a, on which are other buds, 6, c, d. 



Fig. 7. — Metamery in the disposition of the buds. 



Fig. 8. — Dichotomous ramifications of tlie peduncle, having a bud at each 

 extremity. That on the left side seems to be about to divide longitudinally, 

 which will probably give rise to further dicliotoiny. 



Figs. 9-11. — First stages of the development of buds. In addition to the long 

 spicules, there are smaller ones in the form of stars. 



Figs. 12, 13. — Buds at the surface of Tethya lyncurium, from Sicily. 



* 'Arch. Zool. Exp. et Gen.,' viii. (1880) p. 417. 



