376 Kalph S. Lillie 



above band of cells (see Child, loc. cit, Figs. 112, 113). The cella 

 retaiu their large size througbout larvai development, sbowing little 

 ebange in character from tbe early larvai stages (tbree somites) 

 until tbe cümpletion of the defiuitive number of somites. 



Tbe large terminal mesoblast cells of tbe larva are similarly to 

 be rcgarded as tbe descendents of tbe two teloblasts of tbe early 

 embryonie mesoderm (Child, loc. cit. p. 638). Tbe embryonie 

 mesoblast bands take np a ventro-lateral position between the 

 entoderm and tbe ventral ectoilerm of tbe posterior region (Fig. 119). 

 At tbis stage each band is terminated by a single large teloblast. 

 According to Child "visible diftereutiation in the descendents of 

 tbe mesoblasts is very slovr. Tbe cells retaiu their embryonie 

 appearance tili a late stage" (p. 639). Tbe original condition in which 

 each mesoblast band is terminated posteriorly by a single teloblastic 

 cell does not apparently persist long, for in tbe larvai stages the 

 teloblastic region of the mesoblast always contains several large 

 embryonie nuclei, as above described, which exhibit no Constant 

 nnmber or arrangement. Their position, bowever, between tbe 

 posterior attachment of tbe intestine and tbe ventral ectoderm coires- 

 ponds aeciirately witb their position in later embryonie stages as 

 described by Child. Between tbe latest stages described by tbis 

 author and tbe earliest I have examined intervenes a period which 

 has not yet been studied witb reference to tbe character of the 

 mesoblast. There can bowever be no doubt tbat examination would 

 disclose a direct continuity between tbe teloblasts of the later 

 embryonie stages, and tbe posterior imdififerentiated mesoderm cells 

 of the larvai growing zone. 



(b) Formation of tbe Somites. 



Tbe first larvai nephridinm appears in somite IV and becomes 

 recognizable soon after the formation of tbe early septuni which 

 se|)arates tbis somite from tbe one immediately anterior to it. Young 

 larvae tbat bave just terminated the swarming stage (e. g. Piate 22, 

 Fig. 8) thus generally exhibit tbis nephridium in tbe early stages 

 of its formation in the mesoderm of the posterior region. Since all 

 of tbo nephridia arise in exactly the same manner, a description of 

 the changes taking place in tbe posterior mesoderm of such a larva 

 will apply to all of tbe early nepbridial somites. The mesoderm 

 of tbe non-nepbridial somites undergoes closely similar changes — 



