DEVELOPMENT OF THE SIMPLE TISSUES. 539 



quences) again become capable of performing such movements. 

 It has established, further, that this does not apply to old 

 cells. It is found also that the external layers of the cells 

 remain unchanged, and that the central portion alone retracts 

 from the neighbouring parts ; so that the cell becomes con- 

 verted into a vesicle, in the inteiior of which one or more 

 amoeboid cells are contained. 



The account of endogenous cell formation given by Briicke 

 (vol. i., p. 35) is in remarkable accordance with these facts, 

 and the whole is completed by the observation of Oser,* 

 that the endogenously formed cells escape through fissures in 

 the maternal sheath. 



The development of the epithelia and endothelia, after the 

 full description that has been given of them in this chapter, 

 requires no further consideration. 



Rollett has detailed the development of the connective 

 tissues in the second chapter of this work. I need only 

 remark, as the subject is on the tapis, that I consider the 

 origin of fibrillar connective tissue from cell processes to be 

 demonstrated; and that, on the other hand, I think the 

 splitting up of a homogeoeous matrix into fibrils has not 

 been satisfactorily shown to occur. 



Our knowledge of the first traces of the embryonic blood- 

 vessels has reference exclusively to the germ of the Fowl. 



C. F. Wolff was lonor a^o aware of the fact that the blood 



O O 



was developed in the form of islands in the germ disk of 

 the Fowl, and Pander went a step further back when he 

 showed that Wolff's blood islands proceeded from smaller 

 dark islands which make their appearance both in the trans- 

 parent area and in the area opaca. These islands, Pander 

 remarks, elongate, become more slender, communicate with 

 each other by their extremities, and form a reddish plexus 

 with transparent meshes. Baer likewise mentioned Pander's 

 islands, but gave rather a confused account of them, and 

 subsequently Pander's observations fell into oblivion. After 

 the appearance of Remak's works, that author was generally 

 credited with the discovery of these facts; and though he 



* Ibidem, p. 83. 



