330 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
5. Lamellibranchiata. 
Oogenesis and Segmentation in Cyclas.* * * § — Herr H. Stauffacher gives 
an account of the first stages in the segmentation of Cyclas cornea , which 
have not been previously described. The ovum divides into a macromere 
and a micromere, then the micromere divides equally, and its daughter- 
cells likewise, so that the five-cell stage consists of a macromere and 
five micromeres. The mesoderm arises directly from the division of the 
macromere into two equal parts, and is paired from the outset. Ziegler’s 
supposition that the invaginating cylindrical cells of the endoderm 
might arise from one large cell, conspicuous during segmentation, is 
correct. 
Molluscoida. 
a. Tunicata. 
Evolution of Prseoral Lobe.j — Mr. A. Willey, who has lately tried 
to show that the fixing organ of the Ascidian larva is both morphologi- 
cally and actually a prseoral lobe, suggests that the relations it holds in 
Protochordates and Vertebrates, as compared with those that obtain in 
Invertebrates, is due to its complete emancipation from the central ner- 
vous system ; simultaneously with this emancipation there has been a 
change in the formation of the lobe, so that the nervous elements of the 
Invertebrate praeoral lobe are entirely lacking in Vertebrates, while 
the mesodermal element is represented by the prmmandibular head- 
cavities which give rise to most of the eye-muscles. 
Genital Products in Polyclinidse.J — M. Caullery gives a brief 
account of his observations on the degeneration of the genital products 
in the Compound Ascidians Circinalium concrescens and Polyclinum 
luteum. He is led to adopt the view of Kowalewsky, that, in the germi- 
nation of the Polyclinidae, the genital organs of the bud arise directly 
from those of the parent. He has, in fact, observed in them, at all stages, a 
continuous genital tube which appeared to him to arise directly from the 
indifferent epithelial portions of the genital apparatus of the parent. 
As, on the other hand, he has observed a very distinct rudiment of a 
genital cord in the oozooid there would seem to be a continuity between 
the genital products of all the individuals derived from one egg. 
Metamorphosis of Distaplia magnilarva.§ — Prof. W. Salensky has 
studied this form in order to test his theory as to the origin of metagenesis 
in Tunicates. He describes the structure of the larva, the origin of the 
primordial bud, and the degenerative metamorphosis. Comparing the 
degeneration of Distaplia with that of Botryllus , the author notes that in 
both cases the phenomena consist in the disassociation and liberation of 
cells ; that some of the freed cells, most in fact, pass into the cavity of 
the bud or nutritive stolon to become mesenchyme elements ; that others 
(the muscular cells of the body-wall in Distaplia and of the tail also in 
Botryllus ) serve as nourishment for phagocytic wandering cells ; that a 
third set — a small remainder — form a mass of debris which is probably 
* Vierteljahrsschrift Naturf. Gesell. Zurich, xxxviii. (1893) pp. 361-70 (4 figs.). 
f Anat. Anzeig., ix. (1894) pp. 329-32. 
X Comptes Rendus, cxviii. (1894) pp. 666-8. 
§ Morphol. Jahrb., xx. (1893) pp. 449-542 (5 pis., 1 fig.). 
