450 Miscellaneous. 



to be incorrect ; at least, in the Sepiolce this membrane is furnished 

 with a micropyle, and must therefore be considered a chorion. 



The ova of the Sepiolce are completely transparent. Their de- 

 velopment lasts from thirty-four to thirty-five days. The chorion 

 undergoes modifications in proportion as the foetus is developed : it 

 increases in size and its thickness diminishes ; moreover it changes 

 its form, and, from being ovoid, becomes spherical towards the close 

 of the development. 



The author distinguishes three periods in the embryogenic deve- 

 lopment of the Sepiolce : the first, which extends to the completion 

 of the blastoderm, lasts ten days ; the second, during which the 

 organs appear, lasts only five da^vs ; and the third, during which the 

 organs previously formed are gradually developed, lasts nineteen or 

 twenty days, and terminates with the exclusion. 



First period. — The phenomenon of partial segmentation has not 

 been studied from its commencement by the author. The youngest 

 ova which he had under his hands already had the pointed pole of 

 the vitellus covered with embryonal cells. Between this first rudi- 

 ment of the blastoderm and the chorion some drops of protoplasm 

 seem to correspond to the supposed directive cells {Richtungs- 

 blascheii) of the Gasteropods, Insects, &c. The nucleated cells of 

 the blastoderm form at first a single layer ; they seem to multiply 

 by division. Towards the end of the second day, the blastoderm is 

 extended so as to cover two-elevenths of the vitellus. 



On the third day the blastoderm divides into two superposed 

 lamellae. This stratification seems to be the result of a transverse 

 division of the primitive cells. The two lamellae are similar in thick- 

 ness, but in each of them the thickness diminishes towards the 

 margins of the blastoderm. 



In the latter part of this period the edges of the blastoderm gra- 

 dually extend over the vitellus. They envelope the half of it by 

 about the eighth day, and on the tenth they meet at the superior 

 pole of the ovum, enveloping it entirely. The rapidity of the growth 

 in the last two days is explained by the difference of thickness of the 

 different parts of the blastoderm. In fact the lower region, corre- 

 sponding to the future embryo, presents a much greater thickness 

 than the upper part, which is to become the vitelHne or umbihcal 

 vesicle. During the whole of this first period each of the lamellae 

 of the blastoderm is formed only by a single layer of cells. These 

 are capable of executing very marked amceboid movements. 



Second period. — From the commencement of this period the cells 

 of the outer lamella of the superior part of the blastoderm become 

 covered with vibratile cilia, the movement of which causes a rotation 

 of the embryo. At the same time the blastoderm (especially the 

 inner lamella) thickens in its lower region, and the foetus begins to 

 be distinguished from the vitelline vesicle placed above it. Soon a 

 thickening of the blastoderm with an oval outline makes its appear- 

 ance on each side of the body, a little below the equator of the 

 vitellus. These inflations, which belong essentially to the outer 

 lamella, and which from the second day are very distinct, are the 



