THE DEVELOPMENT OF THE EARTH-WORM. 229 



the septa become perforated in many points, the cavities of 

 the primitive somites communicate freely and form together 

 the general " somatic" or " body" cavity. 



Of the horizontal walls of each zoonite the external is 

 placed beneath the ectoderm, the internal encii'cles the epi- 

 thelium of the digestive cavity. The external layer resulting 

 from the splitting of the mesoderm is called the somatic 

 lamina, the inner the splanchnic lamina ; their origin and 

 the part they play in the formation of the body leave no 

 doubt of their homology with the layers of vertebrates, dis- 

 tinguished by the old and somewhat inappropriate terms 

 fibro-cutaneous (Haut-faser-blatt) and fibro-intestinal (Darm- 

 faser-blatt). This is an agreement of the highest theoretical 

 importance, because the analogy in the development of the 

 primitive zoonites, of the somatic cavity, and of the somatic 

 and splanchnic laminae, shows with surprising clearness the 

 close relation between the vertebrates and annelids. 



Now, it is clear that the differentiation of the cephalic 

 germinal streak is essentially the same as that of the ven- 

 tral germinal streaks, and differs only in points of secondary 

 significance. The head cavity is formed by the fusion of two 

 lateral fissures, Avhicli divide the mesoderm into a somatic 

 and splanchnic lamina. But while the zoonites of the trunk 

 generally embrace the whole circumference of the trunk and 

 close in the dorsal median line to form perfect rings, the 

 cephalic zoonite, which from the first is placed above the oral 

 fossa, is unable to complete itself in the same way, because, 

 when its lateral branches direct themselves downwards and 

 backwards towards the ventral surface they meet the first 

 zoonite of the trunk, and hence the cavities of this zoonite and 

 of the head unite. The anterior end of the head segment 

 becomes more and more prominent, and is transformed into 

 a cylindrical process, the upper lip — a kind of proboscis. 



It is evident at the first view, from the chronological order 

 in which the formation of the primitive segments and the 

 splitting of the mesoblast takes place, that the segmen- 

 tation begins in front and gradually proceeds backwards. 

 But it is still necessary to know whether the first zoonite 

 of the trunk or the cephalic zoonite is the first formed, 

 because Semper has attributed great importance and a 

 fundamental significance in the morphology of all articu- ^ 

 lated animals to the fact that, in the development of verte- I 

 bratesand in the organic multiplication of the Naidae, certain | 

 segments of the head appear later than those of the body.^ | 

 The investigation of this point is not easy in the embryos 



' Loc. cit. 



