DEVELOPMENT OF THE SIMPLE TISSUES. 541 



vesicle, and ultimately as many fusiform cells appear to be 

 present as nuclei are seen in optical transverse section. Klein 

 has shown that these cells are constricted off from the inner 

 wall of the vesicles, and falling into the cavity of the vesicle, 

 become blood corpuscles. 



The isolated cell elements are also recognizable in sections 

 of hardened specimens, and from these it appears that they 

 must be regarded, for reasons that have already been given, 

 like the cells of the middle lamina, as descendants of the 

 cleavage mass, which however now migrate or wander into it. 

 It further appears that in accordance with their definite position 

 they must be regarded as belonging to the middle germ lamina. 

 Thus we see that from a cleavage spheroid or an embryonal cell a 

 blood-corpuscle-holding vesicle, or, as we may also say, a vessel 

 constructed upon the type of the capillaries, but completely 

 closed, is formed. The wall of the vesicle is composed of 

 protoplasm, the nuclei of which have increased in number, 

 and the cavity then originates, as vacuolse generally arise. 



According to the description given by Klein, blood corpuscles 

 are developed endogenously in the cells, owing to buds pro- 

 truding from the internal wall of the vesicle, which become 

 constricted at the base, and fall into the cavity of the vesicle. 

 But a second mode of endogenous blood formation also occurs, 

 which is more analogous to the well-known endogenous forma- 

 tion of cells. The central part of a large cell sometimes 

 undergoes conversion into blood corpuscles, so that we have 

 before us a cyst filled with blood corpuscles. Essentially both 

 forms are alike, and in both cases closed and blood-corpuscle- 

 containing vessels originate in single and isolated cells. 



The walls of such vesicles give off projections, which are at 

 first solid, but subsequently become hollow. The free extre- 

 mity of a bud of this kind may again grow out to form a 

 vesicle of one form or the other, so that two cysts communicate 

 with one another, or the buds of different vesicles may inter- 

 communicate, or a bud may open into a vesicle, or the vesicles 

 may open directly into each other, and thus a communicating 

 vascular system originates. The formation of buds still con- 

 tinues after the communicating plexus has been formed. In 

 the tail of the Tadpole, where the new formation of vessels 



