3o8 MENSCH. [Vol. XVI. 



the last segment of the parent stock. Later these fibers are 

 increased in number by additions of tissues from the dorsal 

 dense area of cells (PL XIII, Fig. i8, m.eJ), and at the same 

 time the tissues of this region divide into the somatic and 

 splanchnic layers, so that at a slightly older stage (PI. XIV, 

 Fig. 2i) there is present a dorsal coelomic cavity {coe.) with 

 distinct overlying muscle tissue dorsally (in.d.), while laterally 

 it gradually merges into the more differentiated tissue. The 

 ventral dense area of the embryonic mesodermal cells, seen in 

 PI. XIII, Fig. 1 8 {m.e."), still persists in PI. XIV, Fig. 21 {m.eJ% 

 and it is from this region of cells that the reproductive organs 

 and nephridia will be formed. The nephridia can very frequently 

 be distinguished in a stage of development represented by stolon 

 2, but more frequently they can be clearly defined only in later 

 stages. 



The male reproductive products may be detected in small 

 numbers in a stolon corresponding in external development to 

 stolon 4. A little later these sperm cells become quite numer- 

 ous, and in a transverse section of the second setigerous segment 

 of a stolon of the age of stolon 5 (PI. XIV, Fig. 29, s.p.) they 

 occupy a great part of the body cavity. At a later stage these 

 cells become still more abundant and completely fill the body 

 cavities of the first three setigerous segments, and at the time 

 the stolon is liberated from the chain the spermatozoa have 

 completely ripened. 



The ova may be distinguished in the developing stolon even 

 at an earlier stage than the male reproductive products, and not 

 infrequently in a stolon in which the head is just beginning to 

 appear small ova may already be distinguished in the lateral 

 body cavity. Near the time when the stolon separates from 

 the chain, the ova become so numerous as to completely fill the 

 body and even fill the cavities of the parapodia, after which, by 

 the relaxation of the ventral tissues, caused by the pressure of 

 the increasing ova upon this region, the egg-sac is formed, 

 and the ova from all parts of the body are transferred into 

 this sac. 



