ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 311 



nally with true endoderm. As soon as the septal-ostia are established, 

 the eight Lappentaschen are formed. 



The ectoderm of the tentacles has stronjz longitudinal muscle-fibrils, 

 the endoderm has fine circular fibrils which are sometimes branched ; 

 both are cross-striped. 



In the eight- armed stage mesenchyme cells make their appearance 

 in the supporting substance ; as the development of the larva proceeds 

 they become more and more abundant, and a typical fibrous connective- 

 tissue results. The cells in question arise from both ectoderm and 

 endoderm. 



The degeneration of the tentacles is due partly to basal constriction 

 and detachment, partly to atrophy and shrivelling, helped by phagocytes. 

 The sense-organs are not the modified basal portions of the tentacles, 

 they arise by evagination from the sub-umbrellar margin of the eight 

 Lappentascken, central to the per-radial and inter-radial tentacles. In 

 the free ephyra a connection between optic cells and ganglion-cells may 

 be seen. 



Historical Study of the Zoological Investigation of Hydroids.* — 

 M. Bedot gives a list of the works dealing with hydroids up to 1821. 

 He chronicles the chief classifications proposed, and gives a bibliographic 

 index of the genera and species. 



Development of G-onothyraea loveni.f — J. Wulfert has published 

 the full account of his investigations on the development of this hydroid, 

 very common on the shores of the Baltic. The wandering primitive 

 sex-cells are recognisable soon after the fixing of the planula. They 

 arise from the interstitial cells of the ectoderm, soon pass into the endo- 

 derm of the young stem and hydranths, and afterwards migrate into the 

 ectodermal nucleus of the medusoid bud. In older colonies there is a 

 continual process of new formation in the portions of the stem below a 

 gonangium, and a continued migration of primitive sex-cells. 



The form and position of the ovum in the gonophore are variable ; 

 during its development the egg has no definite orientation. Maturation 

 is typical. Fertilisation may occur by simple apposition of male and 

 female pronuclei, or by a penetration of the compact male chromatin- 

 body into the female pronucleus. 



Two types of segmentation occur, connected by transitional modes. 

 In the one case, a multicellular cceloblastula results, and the endoderm 

 arises by multipolar immigration of blastoderm cells. In the other case, 

 blastoderm cells begin to pass into the interior at the 24-cell stage ; thus 

 segmentation and endoderm-formation are not rigidly separable. 



Sooner or later the segmentation-cavity is filled up with endoderm 

 cells, and the result is a multicellular solid embryo, which was formerly 

 misnamed a morula. Both ectoderm and endoderm include indifferent 

 interstitial cells. 



The free-swimming planula fixes itself, and becomes a flat disc, from 

 the centre of which the hydrocaulus arises. When this is several milli- 

 metres in height it shows at its tip the primordium of the first hydrauth. 



* Rev. Suisse Zool., is. (1901) pp. 379-51"). 



t Zeitschr. f. wiss. Zool., lxxi. (1902) pp. 29G-327 (3 pis.). 



