42 GENERAL ANATOMY OF THE TISSUES. 



become surrounded by an albuminous coat, it is more than probable that 

 whenever, in the body, fat and albumen in the fluid condition come into 

 contact, similar vesicles are produced. 



A peculiar class of elementary vesicles is formed by the elements 

 which occur in the yelk of certain animals. We are best acquainted 

 with them in the yelk of the hen's egg,* in whose proper yelk-substance 

 and yelk-cavity the globules which have been so long known are all vesi- 

 cular, but have not the nature of cells. The membranes of these yelk- 

 vesicles are excessively delicate and consist of a protein compound; the 

 contents are fluid albumen, and, in the globules of the yelk-cavity, there 

 is usually a large parietal fat globule, while in the others there are many 

 smaller and larger ones. The development of these vesicles proceeds, 

 in all probability, from the fat globules as in other elementary vesicles, 

 from which, however, they are distinguished by the fact that they dis- 

 tinctly possess the power of growth, during which their contents undergo 

 metamorphosis, since in many the number of fat globules increases with 

 age. Similar vesicles exist, also, in the yelk of fishes, Crustacea, and 

 spiders, and here, as in birds, they have only a temporary importance, 

 since they are not directly applied to the formation of the embryo, but 

 only serve to nourish it. 



Lastly, free nuclei occur in many localities, either temporarily, where 

 cells are formed immediately round nuclei, as in the chyle, the blood- 

 vascular glands, the Peyerian patches; or permanently, as proper ele- 

 ments of the tissue, in the wall of the thymus vesicles, in the rust-colored 

 layer of the cerebellum, and in the granular layer of the retina. f 



Von Wittich (<De Hymenogonia albuminis,' Regimontanii. 1850), 

 has lately given some information upon the formation of the so-called 

 Aschersonian vesicles. According to Wittich, whenever oil and albumen 

 come in contact, a portion of the oil is saponified by uniting with the 

 alkali of the layer of albumen in contact with it, and this layer being 

 thus rendered insoluble by the deprivation of its alkali becomes precipi- 

 tated and thus forms the Aschersonian so-called haptogen membrane. 

 According to this explanation the process would be purely chemical and 

 not physical, and still less vital. In opposition to this view, how- 

 ever, Harting ('Ned. Lane.' Sept. 1851), observed, not long! ago, the 

 formation of pseudo-cells by the agitation of albumen with mercury, in 

 which case the albumen must be solidified, in the same way as by the mere 

 shaking with water or otherwise (Melsens, in ' Bull, de 1' Acad. de Bel- 

 gique,' 1850. Harting, &c.). Again, if by the bringing together of 



* [It is, however, by no means certain that the yelk-corpuscles of the hen's egg are elemen- 

 tary granules. According to Dr. H. Meckel (Die Bildnng der fiir partielle Furchungbestirnm- 

 ten Eier der Vogel, c., Siebold and Kolliker's ' Zeitschrift,' 1852,) they are altered cells. 



TBS.] 



t [The blood corpuscles of man and the mammalia should be added to this list. See 

 Wharton Jones, 'Phil. Transactions,' 1846. TKS.] 



