960 SUMMARY OF CURRENT RESEARCHES RELATING TO 



according to the author, not only endoderm but mesoderm, though 

 Barrois asserts the distinct definition of the mesoderm as two lateral 

 " cordons " constricted off from the young endoderm. Cavities are 

 observed in the cell-complex above referred to, and these represent the 

 primitive coelom, and are regarded by Vigelius as remnants of tho 

 blastoccel which have enlarged and become modified during the growth 

 of the embryo. 



Meanwhile the ectoderm cells in the oral and aboral halves have 

 multiplied. An annular thickening is observed in the equatorial plane, 

 composed of two rows of cells, one belonging to each half. From the 

 aborul ring of cells the ciliated corona appears to be formed. Two 

 ectodermic invaginations are formed on the oral surface. One of these, 

 lined with high cylindrical cells, is the commencement of the suctorial 

 pit, which serves for the attachment of the larva. Its opening lies a 

 little behind the oral pole. Tho invagination increases in size and 

 becomes sac-shaped, occupying a large part of the body. The second 

 invagination has been repeatedly described as the oral groove (" Mund- 

 furche "). It has, however, nothing to do with the mouth, and is more 

 fitly described as the anterior ectodermic groove. At the aboral pole, a 

 multiplication of ectoderm cells forms a tbird organ, very variously 

 designated, the retractile disc. It intrudes for some distance into the 

 interior of the embryo, as a thick flattened disc. Round it a circular 

 invagination of the ectoderm takes place, forming an oblique groove. 

 The cells at its blind end proliferate and form a peculiar pear-shaped 

 organ. This Vigelius describes in some detail, and regards as a gland, 

 which lias possibly a function in connection with the formation of the 

 tegumentary skeleton. All these structures increase in size and are 

 slightly modified. Shortly before escaping, the embryo undergoes 

 modifications of form very different from those of the larva. 



The external characters of the larva are then described. The 

 degeneration of the long coronal cells and similar changes are noted, 

 and lastly the metamorphoses of the larva. The author's results, of 

 which a completion is promised, support Barrois' conclusion that the 

 development is in no way a metagenesis, but a true metamorphosis in 

 which the organs of the sessile adult are directly developed from 

 definite organs present in the larva. 



Metamorphosis of Fresh-water Polyzoa.* — Herr A. Ostroumoff 

 finds that the cells of the ectoderm at the hinder pole of the larva are 

 much higher than those on the rest of the surface, and he is inclined 

 therefore to regard tbis area as a functionless rudiment of the sucker, 

 which is found in all marine Ectoproctous Bryozoa. 



The whole course of metamorphosis in Alcyonella may be divided 

 into two stages ; in the first, which is common to all Ectoprocta, the 

 mantle bends over on the basal side ; in the second, which is found in 

 fresh-water forms only, the basal side with the edges of tbe mantle are 

 invaginated, so that the ascending portion of the mantle-cavity forms a 

 canal, the walls of which soon fuse, and from which the body-wall of 

 the primary zocecium is alone developed. 



* Zool. Anzeitf., ix. (188G) pp. 547-8. 



