330 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are brought nearer to the superior pole of tlic egg. The layer wliich 

 in all Cephalopods forms the wall of the outer yolk-sac, appears to 

 the author to be simply formed of mesodermal cells, of which it 

 would appear to be a direct continuation. The various facts which 

 Dr. Ussow has observed lead him to think that in the Cephalopoda 

 the mesoderm is not folded off from the ectoderm, but simply arises 

 from the transverse division of the cells of that layer. Later, the 

 diameter of the unilaminatc central portion decreases considerably, 

 while the median zone grows both centrifugally and centripetally. 

 The cells of the ectoderm at first vary in form and size in different 

 parts of the embryo ; later on they all become short epithelial cells ; 

 but it is not till the ninth or tenth day that they are to be sharply 

 distinguished from all the rest, and they are then cylindrical in form. 

 The mesoderm grows in two directions, towards the central portion 

 of the germ and the equator of the egg. 



Contrary to the opinion of Kolliker and others, the author is con- 

 vinced that all the Cephalopoda begin to develope from the dorsal 

 side, and not from the hinder end of their body. Further observations 

 are promised. 



Development of the Oyster.* — Dr. E. Horst points out that the 

 groove or depression described by Davaine and Lacaze-Duthiers is 

 the invagination of the embryo, and that the dorsal depression regarded 

 by Brooks as being the opening of the intestinal tube is really the 

 .shell-gland-iuvagiuation. These two inpushings, possessed by the 

 oyster at one and the same stage, are almost equally well developed ; 

 later on the ventral side becomes a little pushed out so as to form a 

 kind of foot. The abdominal cavity is formed by the separation of 

 the ectoderm from the endoderm. The author confirms the doctrine 

 of Salensky and Hatschek that the first rudiment of the shell is an 

 unpaired formation, and he thinks that this is true of all Mollusca ; 

 Carbonate of lime is very early deposited in the shell. The white 

 spat becomes black spat by the deposition of pigment at different 

 points in the body of the larva. On the ventral face there is a 

 button-like thickening of the ectoderm, which is probably the com- 

 mencing rudiment of the otocyst. 



Abortion of Reproductive Organs of Vitrina.t — F. d'A. Furtado, 

 on examining seven specimens of Vitrina from the Azores, found 

 that there was not the least trace of any reproductive organs, and 

 Professor L. C. Miall confirms the observation as regards three other 

 specimens sent to him. Abortion of the reproductive organs has 

 been observed in animals infested by parasites, e. g. in stylopized bees, 

 in Lymncea stagnalis when attacked by Trematodes, and in female 

 hermit-crabs attacked by Rhizocephala. The complete abf)rtion of 

 the parts, writes Professor Miall in the remarkable case described by 

 Mr. Furtado, distinguishes it at once from the many cases of real or 

 supposed functional defect met with in hybrids. 



* Zool. Anzel^., v. (1882) pp. 160-2. 



t Ann. and Mag. Nat. Hist., ix. (1882) pp. 397-9. 



