82 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



derivation. It is not difficult, then, to understand from this condition 

 how, in forms having a closed blastopore, the coelomic mesoblast may 

 arise from the entire length of the linear blastoporic area, and we 

 may further see some significance in the statement by Sedgwick 

 (1887) that in the onychophoron Pcripatopsis the mesoderm bands 

 in their forward growth are augmented by cells derived from the 

 lips of the blastopore. In the more specialized types of arthropod 

 development evidence of teloblastic generation of the mesoderm is 

 entirely lost, or at least obscured, and the whole of the mesoderm 

 appears to be a direct product of the germ band closely associated 

 with the endoderm. In its full development the arthropod mesoderm 

 surrounds the blastopore anteriorly, since in the adult the lateral bands 

 of the cephalic mesoderm may be continuous from side to side in 

 front of the mouth. 



Segmentation of the mesoderm and the subsequent formation of 

 coelomic sacs take place in the early embryonic stages of many Crus- 

 tacea and Arachnida almost as completely as in the Onychophora and 

 Annelida, but in the myriapods the coelomic sacs are small, and in 

 the insects they are for the most part represented only by cleavage 

 spaces in the lateral parts of the mesoderm. In all cases, however, 

 the walls of the sacs break down, except such parts of them as are 

 retained in the formation of certain organs of coelomic origin, and 

 the haemocoele is restored as the definitive body cavity. Probably 

 all muscle tissue of the arthropods is produced from the coelomic 

 mesoblast ; though some writers have claimed that certain muscles 

 are produced directly from the ectoderm, the evidence is open to 

 question and needs closer scrutiny (see Needham, 1937). 



PRIMARY AND SECONDARY SOMITES 



There is ample reason from arthropod ontogeny for believing that 

 the arthropods have been derived, as have the annelids, from primi- 

 tively unsegmented ancestral forms in which metamerism first ap- 

 peared as a direct subdivision of the primary body region into a small 

 number of somites, and that the subsequent increase in the number 

 of somites proceeded secondarily from growth in a subterminal 

 zone of undifferentiated cells. This dual method of somite produc- 

 tion is recapitulated in the embryogeny of some of the arthropods, 

 and teloblastic growth is of frequent occurrence in postembryonic 

 development. 



In the Trilobita it seems very probable, as contended by Iwanoff 

 (1933) and Schulze (1936), that the so-called head represents the 



