228 NIKOLAS KLEINENBERG. 



important event, on which is in great part founded the typical 

 structure of the body both of Annelids and Vertebrates, I 

 shall only say a few words, because I do not wish to enter 

 here into the consideration of the particulars of histogenesis ; 

 I do not know how to do better than to repeat the beautiful 

 and mosf exact explanation given by Kowalewsky of the 

 process in Euaxes and Lumhricus ruhellus. 



In L. trapezoides , as in the above-named Oligochseta, the 

 successive division of the mesodermic cords into segments as 

 primitive zoonites precedes the splitting of the mesoderm. 

 It appears that this division happens in L. trapezoides at a 

 little later period than in the other species, because when, the 

 first traces of it can be discerned, when, that is to say, the 

 finest transverse lines of demarcation appear between suc- 

 cessive portions of the mesodermic cords, these are already 

 very thick and contain two, three, and more layers of cells ; 

 the space which divides the streaks in the median ventral 

 line is, on the contrary, still very wide. Hence are formed 

 two parallel rows of transversely elongated, rectangular 

 plates. Afterwards each plate becomes split by a horizontal 

 fissure, so that the mesoderm is divided into two unequal 

 lamina, of which, unlike what we have noticed in the split- 

 ting of the cephalic germinal streak, the external is much 

 thicker than the internal, which only consists of a single 

 layer of cells (PI. IX, fig. 13). And as the splitting does not 

 pass beyond the limits of the primitive zoonite, nor reach to 

 its boundary line, the cavity remains surrounded on all sides 

 by mesoderm cells; each primitive zoonite is transformed 

 into a compartment, or rather, into a four-sided prismatic 

 case, with a central cavity whose external wall is thickened, 

 while the internal consists of a single layer of cells. The 

 anterior vertical wall of each compartment adheres firmly to 

 the posterior wall of the segment in front of it, and thus 

 are formed the septa, stretched between the body-wall and 

 the intestine. They are thus at first each composed of two 

 layers belonging to two adjoining zoonites ; then, in conse- 

 quence of the strong tension which they have to sustain, the 

 cells group themselves into a simple, very thin membrane, 

 which is not placed vertically to the long axis of the embryo, 

 but goes obliquely from behind forwards. Hence, in almost 

 all perfectly vertical transverse sections, are seen on each side 

 two separate cavities ; the ventral is the posterior part of a 

 segment and the dorsal the anterior part of the following 

 segment; the row of cells which divides them is the oblique 

 section of the septum. Not rarely two cavities are formed in 

 the same primitive compartment^ but they soon unite. Later, 



