NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS IQ 



the facts concerning the ontogenetic origin of the annehd mesoblast 

 apparently can be expressed in the simple statement that mesoblastic 

 tissue may be formed by internal proliferation from any part of the 

 blastoderm, and may, therefore, be both "ectodermal" and "endo- 

 dermal." The mesoblast of the first three quartets of the blastula 

 (see Torrey, 1903) gives rise to the so-called larval mesoblast, or 

 mesenchyme ; from the fourth quartet ordinarily arises the coelomic 

 mesoblast, or mesoderm. That these two groups of mesoblast cells 

 primarily have the same morphological status is indicated by the fact 

 that in the Platyhelminthes they do not become differentiated into 

 separate tissues. Surface (1907), who first followed the divisions of 

 the 4d cell in a flatworm, shows that in Planocera the 4d blastomere 

 gives rise to both endoderm and mesoblast as it does in the annelids, 

 since, of the two cells of the first division, 4d^ (fig. 5 H) forms the 

 endoderm (I, End), and 4d^ gives rise to two lateral groups of scat- 

 tered mesoblast cells (I, Msb'^), which are at first quite distinct from 

 the mesoblast of the second quartet {Msb~), though eventually they 

 intermingle with the latter to form the parenchymatous tissue of the 

 adult. In Planocera the usual endodermal "macromeres" degenerate 

 and almost the entire endoderm proceeds from the 4d^ cell. Finally, 

 we may correlate the "double origin" of the annelid mesoblast with 

 the production of muscle tissue from both the ectoderm and the 

 endoderm in the Coelenterata. 



From the condition in the Platyhelminthes, it becomes evident that 

 the primitive mesoblast was a parenchymatous mass of undiffer- 

 entiated cells occupying the haemocoele, which had been proliferated 

 internally from both the ectoderm and the endoderm. In the unseg- 

 mented ancestors of the annelids, the ectodermal mesoblast must have 

 formed a primary somatic muscular system, represented by the larval 

 musculature of modern annelids, which is derived from the ectodermal 

 quartets of the blastula. The principal part of the parenchyma, there- 

 fore, came to be that part of the mesoblast proliferated in the posterior 

 part of the body, chiefly, or usually, from the 4d cell of the fourth 

 quartet. The persistent parenchyma thus became the embryonic 

 middle layer known specifically as the mesoderm. 



Since the most important result of metamerism is the production 

 of a mechanism of movement based on the division of the body into 

 consecutive motor units, it can scarcely be questioned that meta- 

 merism had its origin as an adaptation to more effectivt body move- 

 ment. Inasmuch as the evidence from embryonic development shows 

 that metamerism originates ontogenetically in the ectoderm and its 

 derivatives, and is secondarily imposed upon the mesoderm, we may 



