344 NIKOLAS KLEINENBERG. 



of this. The secondary groove may, perhaps, be compared 

 to the posterior fissure of the spinal cord of vertebrates, but 

 certainly not to the primitive medullary groove. 



Here I will end, merely adding further, that at the time 

 of the difFerentation of the first cells of the medullary cords 

 the muscular fibres appear at their sides. The rudiments of 

 the segmental organs resemble those of Euaxes, represented 

 by Kowalewsky, and do not develop from the septa, as the 

 same author states that they do in Lumbricus ruhellus. I 

 confess also that I should not have hesitated to describe them 

 as invaginations of the ectoderm, if the very clear figures in 

 the above-quoted work had not obliged me to investigate the 

 the subject from this point of view. Of the formation of the 

 colossal fibres, which Kowalewsky believes to be homologous 

 with the notochord of Vertebrates, I know nothing, but 

 what Semper describes as the notochord of the Naidini is 

 certainly nothing but the cells of the mesodermic sheath, 

 surrounding the nervous chain. 



It is not possible to overlook the great similarity between 

 the development of Annelids and Vertebrates, especially in the 

 formation and transformation of the germinal streak. There 

 would be no great inaccuracy in saying that the belly of Anne- 

 lids is homologous with the back of Vertebrates, were there not 

 serious divergences shown in the development of the neuro- 

 muscular apparatus, which certainly are not diminished by 

 the discovery of the independent origin of the cephalic 

 ganglion. I, however, believe that every well-recognised 

 fact, although it may be such as to appear to open an abyss 

 between two so-called types, is in reality a step in advance 

 towards the establishment of the unity of the organisation 

 of the animal kingdom. I must defer general considerations 

 to a second part of this work, in which I shall treat of the 

 further development of the Earth-worm, and more especially 

 of the formation of the tissues. 



