390 L. DONCASTEU. 



cavities and tlic mesoderm associated with them are formed 

 very early iu embryonic life, while the division of the rest of 

 the body does not take place until the animal has, in most 

 important respects, the structure of the adult. The separa- 

 tion betAveen head and body is also much greater than 

 between the body and tail, for while the latter are at first 

 identical in essential structure, and iu the adult dilfer only in 

 the absence of the alimentary caual in the tail, ^^et the head 

 is from the first widely different from the body, and becomes 

 increasingly so as development proceeds. 



A further point of importance is the mode of origin of the 

 two transverse septa, and here again there is no agreement. 

 The anterior is produced by the meeting and fusion of the 

 somatic and splanchnic mesoblast at a time when differentia- 

 tion of tissues has not begun and the mesoblast is still 

 continuous with the hypoblast; while the posterior appears in 

 close connection with the genital cells, and is not formed by 

 the whole thickness of the mesoderm, but probably only by 

 the cellular envelopes of the genital cells, and certainly by 

 the splanchnic layer exclusively. 



The most probable origin of the posterior septum is hinted 

 at by Grassi when he suggests that its object may be to 

 separate the male genital organs from the female, and this 

 view certainly gains support from its formation at the time 

 of the passage of the genital cells across the body-cavity. 

 From its mode and time of origin it seems reasonable to 

 regard the posterior transverse septum as cssentiall}'' part of 

 the reproductive organs, and not closely connected with the 

 general plan of the anatomy; but if this be the case it is 

 necessary to a.ccount for the presence of the anterior septum. 

 The most probable reason for its existence would be the 

 early separation of tlie anterior part of the mesoderm for the 

 production of the impoi-tant head muscles, and since at the 

 time of its formation in the embryo the body-cavity is widely 

 open, this can only be done by folding off the front end as a 

 separati; space, enclosed in its own mesodermal walls. From 

 the time of its separation the development of the head 



