PEROPHORA ANNECTENS. 53 



migration of cells takes place into the plug of test that 

 fills the cavity of the ectodermal invagination. These 

 cells are always strongly contrasted in their appearance 

 with the ectodermal cells, and are entirely similar to cells 

 of the kind already described, which are abundant in the 

 blood spaces of this region. Although none of these 

 cells have been found in the process of passing through 

 the ectoderm at this point, it is still quite possible that 

 such migrations may have been taking place without 

 having been detected. All my sections that have shown 

 this stage of development have been rather densely stained 

 in this region. 



It is thus seen that my results add one more instance 

 to the two furnished by Salensky and Kowalevsky, in 

 which cells of the tunicate test are not derived from the 

 ectoderm but from mesoderm, or rather in the case here 

 presented, from cells derived from mesoderm; for that 

 such is the origin of the original cells of the blood in 

 Tunicates is well known; the source from which the 

 blood cells are renewed in adult life is, however, not so 

 well known. It is quite certain, from the instances of 

 division of some of them, as is shown 'fn. fig. ^6a, that 

 they are the source of their own renewal — that some of 

 them, at least, alwa3'S retain the power of reproduction. 

 All the cells of the blood are frequently spoken of by 

 writers on the tunicate morphology as " mesenchyme cells 

 swimming in the blood plasma," e. g., Seeliger, '82, p. 



405- 



I must mention here that since completing these ob- 

 servations on this point, I find that Kowalevsky himself 

 seems to have seen the same migration of cells from the 

 blood into the test in Perop/io7^a Listeri, twenty years ago. 

 Thus he says: " Souvent encore, ou peut reussir a voir 

 des globules sanguins hors de la cavite des stolons, contre 



