6() NOTES AND MEMORANDA. 



foi'wai'ils by means of two buttons, and is prevented from falling ont 

 by proper guides in the slit of the base-plate. 



When the long side of the wedge is contiguoiTS to the edge of the 

 beaked piece d (Fig. 2), the first line on the nonius coincides with the 

 zero point of the scale. 



To measure the thickness of a covering glass, the wedge is drawn 

 back till the object to be measured can be placed on the edge of the 

 piece d (Fig. 3). Then it is moved back again, pressing it slightly 

 against the scale until a check is felt to the motion. The first line on 

 the nonius will now no longer coincide with the zero point on the 

 scale (Fig. 3). The number of divisions denoted by figures gives the 

 whole millimetres, the number of smaller divisions the tenths of a 

 millimetre, and the nonius the hundredths of a millimetre. 



In using the instrument, care should be taken that it is free from 

 dust, and that the motion of the wedge is easy. 



By this instrument, when neatly and correctly made, the most 

 exact measurements can be taken with a rapidity and ease that even a 

 well-made screw micrometer will not admit of. 



Origin of the Sexual Products in Hydroids. — J. Ciamician has 

 made a series of careful observations * on the exact mode of origin of 

 the ova and spermatozoa in two genera of Hydroida, and his results 

 are altogether opposed to the theory of Van Beneden, according to 

 which the ectoderm may be looked upon as the male, the endoderm 

 as the female germ-lamella. In Tuhularia mesemhryanlhemum the 

 reproductive organs are sporosacs, and arise as bud-like processes 

 composed of ectoderm and endoderm. The ectoderm at the distal 

 end of this bud undergoes a process of invagination, and the bottom 

 of the sac thus produced growing distalwards, forms from its 

 endoderm the spadia of the sporosac, from its ectoderm the ova or 

 sperm-mother-cells. The generative products of both sexes are 

 therefore products of the ectoderm. 



In Eudendrium ramosinn the ectoderm on one side of the female gene- 

 rative bud undergoes proliferation, and pushes the endoderm towards 

 the opjiosite wall : one of the ectodermal cells thus pushed in, enlarges 

 greatly and produces an ovum, which is finally enclosed, by the com- 

 pletion of the process of virtual invagination, by a double layer of 

 endoderm and a single one of ectoderm. So that in this case also, 

 the ova are ectodermal products. 



In the male gonophore of the same species, the case is quite 

 different. Certain of the cells of the endoderm — the sperm-mother- 

 cells — enlarge greatly, and their nuclei undergo extensive multiplica- 

 tion : as growth proceeds they become completely overarched by the 

 neighbouring endoderm cells, and finally come to lie between the two 

 layers, often having the appearance of belonging rather to ectoderm 

 than to endoderm. Their contents become converted into sperma- 

 tozoa, which are thus endodermal products. 



There is, therefore, almost every possible variation in the origin 

 of the generative products among the Hydrozoa ; in Tuhularia 

 (Ciamician) and in Hydra (Kleinenberg), both male and female 

 * ' Zcitsch. f. wiss. Zoo].,' vol. xxx. p. 501. 



