DEVELOPMENT OF THE SIMPLE TISSUES. 543 



out the whole system. This process is quite in accordance 

 with the general principles of development. Moreover, in the 

 first rudiments of the middle germinal layer no instance is 

 known of cells coalescing to form a cellular structure. In all 

 epithelia, as well as in all endothelia, we see cell division only 

 so occurring that from one cell two or more cells arise ; these, 

 however, never separate from one another, but cementing 

 substances form between them, which indicates the discon- 

 tinuity of the individual cells. The same views must be held 

 in regard to the originally homogeneous protoplasmic tubes. 

 I must once more adduce the example mentioned in the pre- 

 face, that the vessels originally appear as if made like a cannon 

 tube, but that they subsequently seem as if constructed on the 

 plan of a chimney. 



We know very little as to the mode and place of origin of 

 the blood in the embryo after the completion of the first rudi- 

 ments of the vessels have been laid down. Reichert* main- 

 tained that the blood is developed in the liver ; but no satis- 

 factory evidence of this has been adduced. Again, the view 

 entertained by Neumann and Bizzozero as to the origin of the 

 blood in the cancellous spaces of bones cannot be held to ex- 

 plain the origin of the blood during the earliest period of 

 development, since such cancellous tissue is only formed at a 

 later period. At present it is not known whether generally, 

 and if so, how soon, the medullary spaces act as centres for 

 the formation of the blood. For the earlier stages of develop- 

 ment, lastly, we can scarcely regard the lymphatic glands as 

 sources of the colourless blood corpuscles, since, as Sertoli ( 

 has shown, the first traces of these are only formed in embryoes 

 at a later period of development. 



Before I proceed to describe the development of transversely 

 striated muscular tissue, I must add a few words in regard 

 to its structure, which did not find a place in chapter vi. 

 Transversely striated muscular fibres are fusiform or cylin- 

 drical, with blunt or pointed extremities. Their thickness 

 varies to an extraordinary extent, being sometimes even visible 



* Entwickelunysyeschi-chte, etc. 



f Wiener Sitzungsberichte, Band liv., 18G6. 



