CHAP. X,] 



THE DEVELOPMENT OF THE CAT. 



331 



said) not yet positively ascertained, but the other cartilages of the body 

 arc undoubtedly of mesoblastic origin. The walls of the primitive 

 cartilage cells thicken and form the intercellular matrix, acquiring 

 at the same time the special qualities of cartilaginous tissue. The 

 thus-formed matrix may remain clear and structureless, or may 

 become fibrillated, as in the case of fibro-cartilago. 



Bone is a substance which is never directly formed from the in- 

 different embryonic tissue, but requires for its development the 

 pre-existence of either cartilage or connective tissue. The process 

 of its formation in these substances has been already noticed.* 



Epithelial tissue is the most distinctly and permanently cellular of 

 all the tissues of the body, and it arises directly from the cells of 

 the epiblast and hypoblast, with the exception of the endothelium 

 of the vessels and pleuro-peritoneal cavity, and some other parts 

 which are derived (as already said) from the mesoblast. The epen- 

 dyma of the cerebral ventricles is the persistent epiblast of the lining 

 of the medullary groove of the embryo. Ova and spermatozoa may 

 be considered as special modifications of epithelial tissue. 



Blood appears to originate within cells derived from the meso- 

 blast, f either by a multiplication of their nuclei and the acquisition 

 of a red colour by the protoplasm around each nucleus, or else as a 

 sort of deposit within the cells. However originating, the primitive 

 corpuscles when formed become separated from one another by a 

 process of vacuoiation J within the cells. The cells then enlarge 

 and send out processes which unite with those of other cells. The 

 walls which separate the ca%dties of such united processes then dis- 

 appear, so that their cavities communicate, and thus blood and blood- 

 vessels are simultaneously formed. 



The primitive red corpuscles are nucleated, and larger than 

 those which subsequently arise, which latter, together -with the 

 white corpuscles, seem to be formed mainly by the spleen, the 

 nucleated red corpuscles disappearing and being replaced by the 

 smaller, flattened non-nucleated corpuscles, during embryonic life. 



Muscular tissue, though by no means clearly cellular in its fully 

 formed condition, is said to have a distinctly cellular origin, being 

 formed by direct transformation of embryonic cells as follows : § the 

 cells elongate and acquire an investing membrane and often pointed 

 ends. The nuclei multiply, and the contained protoplasm of the cells 

 gradually acquires its striated character. The investing membrane 

 becomes the sarcolemma, and the scattered nuclei become the 

 corpuscles. The unstriped muscular fibres originate simply by the 

 lengthening out and flattening of cells, which acquire pointed ends 

 and an elongated nucleus. 



Nervous tissue is, as has been said, mainly derived from the epi- 



* See ante, p. 20. 



t A peiiect agreement has not yet 

 been arrived at as regards their mode of 

 origin. See Balfour, Quarterly Journal 

 of Microscopic Science, July, 1873 ; and 



Schafer's Proceedings of the Eoyal 

 Society, 1874. 



t See ante, p. -329. 



§ See Wilson Fox, Thil. Trans., 1866. 



