52 



usually compressed from lying one upon the other. Soon after 

 the eggs are deposited, the division of the yolk commences, but 

 does not proceed so regularly as in some ova. Each division 

 contains a transparent dot. On the twelfth day, each yolk, 

 being provided with cilia, has a revolving motion. This 

 movement has been noticed in so many ova of different classes 

 of animals, that it may be considered as the rule for all embryos. 

 Mr. Desor supposes it to commence as soon as the division of the 

 yolk ceases. It varies in its rapidity, is without any special direc- 

 tion, and never stops. At the fifteenth day, when examined with 

 a high power, the yolk presented the appearance of two zones, 

 the outer of which was furnished with cilia, and was of a lighter 

 color than the inner. On the sixteenth day, with a power of two 

 hundred and fifty diameters, the inner body was seen to be moving 

 within, by means of proper cilia, similar to those of the outer. 

 Some days afterwards this inner body assumed the appearance 

 of an embryo worm. On pressing the envelope it tried to escape 

 from it, and succeeded, moving freely about, and dragging after it 

 the remains of the outer coat. This appendage had no analogy 

 to the placenta or the membranes of other embryos, but was 

 exclusively composed of cells of the original yolk. Mr. Desor 

 remarked upon the extreme singularity of this double and inde- 

 pendent motion of the two parts of the ovum. The motion of 

 the inner, he said, seemed to be voluntary, while that of the 

 outer clearly could not be, and resembled greatly that of the 

 spores of Confervse. The discovery of the existence of these 

 two movements is something new in Embryology. Mr. D. made 

 some remarks, which he said he intended to present hereafter in 

 a more extended form, on the nature of the fluid in which the 

 yolk of the Nemertes floats. It exists probably in all ova, but 

 its true nature has hitherto been mistaken by embryologists. He 

 proposes for it the name of the ^^Biogen liquid.''^ In the course of 

 his remarks Mr. D. said that embryologists had laid too great 

 stress on particular days as the epochs of certain stages in the 

 development of ova. These are never precisely uniform in the 

 date of their occurrence, but depend somewhat upon outward 

 circumstances, such as the temperature, for instance. 



Prof. Wyman exhibited specimens of various orders of 

 the Ants which inhabit the gigantic Ant-hills of Africa, the 



