388 WILSON. [Vou. If. 
ParT III. 
PAGE 
XI. Relations of the head [prostomium] and trunk ..............0ec00ee 429 
I Concrescence and thes blastopore) s-yiesie spieminienn setae ioe ete See eierieiele 435 
SCM Lheitrochosphere and thesteloblasts meer micii-inc lest cieieeletaleeeeietere 439 
UV er Mesoblast and coclommcceretayehelayareraieleletsielclehetate sistaletaraimicicieie ateretoieieietneieiots 442 
PART I.— INTRODUCTORY. 
I. Tue following paper contains the results of an examina- 
tion of the development of the earthworm that has occupied 
my attention at intervals for a considerable time. I shall not 
attempt to give an exhaustive account of the development, and 
I must leave unsolved a number of important problems, some 
of which can perhaps only be fully elucidated by the study of 
other and more favorable material. The results already attained 
differ widely, however, from generally accepted views in respect 
to many important phenomena and clearly show the need for a 
re-examination of many features of annelid development, even 
in the case of forms that have already been carefully studied. 
It is hardly necessary to call attention to the historical im- 
portance of Kowalevsky’s pioneer researches on the embryology 
of Lumbricus and Euaxes (No. 27), or to the interest of the 
questions suggested by Kleinenberg’s later and more detailed 
studies (No. 28). Both these works have played an important 
part in the advance of comparative embryology, and must 
always command the admiration of morphologists. Both were 
nevertheless far from exhaustive ; and later investigations have 
left our knowledge of the subject not only incomplete, but as 
I shall endeavor to show in the sequel, obscured by a false 
conception of some of the most important phenomena of de- 
velopment. 
The more important results of my work may be briefly sum- 
marized as follows :— 
(1) The cleavage is unequal and variable, and results in 
the formation of a blastula containing a large blastoccel. The 
gastrula is formed by embolic invagination. The blastopore, 
which at first occupies the entire ventral surface, narrows toa 
slit-like form, its longer axis coinciding with the long axis of the 
adult body; it then closes from behind forwards, its foremost 
portion persisting as the mouth. The germ-bands are already 
established at the time of the invagination and lie at the sides 
