No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 409 
lateral vessels make their appearance near the outer or dorsal 
edge of the mesoblastic plate (Fig. 72). They develop in pre- 
cisely the same way as the ventral vessel, first appearing as 
spaces between the splanchnic mesoblast and the archenteric 
wall, but ultimately acquiring walls of their own. 
In the posterior region these vessels lie parallel to the ventral 
vessel, quite on the ventral side of the embryo. On tracing them 
forwards they are found to assume a lateral position, and finally 
to pass towards the dorsal side, where they finally unite in a 
single vessel which runs forwards in the median line (Fig. 51). 
The arrangement of the longtitudinal trunks at this period is 
almost precisely like that of many adult tubicolous Polychzeta 
(Amphitrite, Melinna, Lantice; see Meyer, No. 35, Taf. 23); and 
Beddard (No. 4) has shown that the dorsal vessel remains more 
or less completely separated into two parallel trunks in a number 
of Oligochzeta (Megascolex, Microcheta, Acanthodrilus).  Bed- 
dard is inclined to consider this the primitive mode of develop- 
ment — a view which, however, seems at present to rest on in- 
sufficient evidence. 
The dorsal vessel is formed by the progressive backward 
concrescence of the two lateral vessels —a process which on the 
whole keeps pace with that of the mesoblastic bands and is only 
finally completed near the time of hatching. The vessels lag 
somewhat behind the remaining mesoblast, however, so that in 
the posterior region they still lie at the sides when the mesoblast 
has entirely surrounded the archenteron, as in Fig. 72, Pl. XX. 
The cells of the mesoblast lying above the lateral vessels seem 
to pass upward in large part by migration, and only become 
arranged in definite somatic and splanchnic layers at a later 
period. 
The first of the circular vessels to appear are the circum- 
cesophageal vessels or pseudo-hearts, which are developed in 
the splanchnic mesoblast in connection with the unpaired an- 
terior part of the dorsal vessel, and as far as I have been able to 
observe are surrounded with definite walls, from the start. The 
circular intestinal vessels develop considerably later, and I have 
never been able to distinguish them until after the complete 
fusion of the lateral longitudinal trunks to form the median 
dorsal vessel. 
