66 J. F. McCLENDON. 



are the mesoblastic rudiments of the mandibles. This completes 

 the rudiments of the nauplius appendages. After the close of 

 the blastopore the post nauplius segments are laid down by 

 teloblastic growth at the posterior end, and the nauplius is pushed 

 (compressed) forward, carrying the rudiments of the second an- 

 tennae and mandibles forward (Figs. 47-48), and causing the 

 three pairs of appendages to lie closer together. In stage D 

 (Figs. 456) the appendages begin to grow out and at the same 

 time the muscle cells elongate into fibers. I think it more profit- 

 able to follow these latter backward in development, as it seems 

 doubtful whether they have a single or a double origin. Observe 

 the muscle cells in Fig. 45 elongating radially and attached peri- 

 pherally to the rudiments of the appendages. In Fig. 44 (Stage 

 C] the muscle cells (one shown at in] are just beginning to 

 differentiate from the mesoblastic rudiments of the appendages, 

 and two of them have begun to elongate (compare Fig. 50, ;//). 

 The question arises whether all or only some of these muscle 

 cells arose from the mesoblastic rudiments of the appendages. 



Just after the closure of the blastopore a few cells similar to 

 these muscle cells are seen considerably removed from the 

 mesoblastic rudiments of the appendages (Fig. 41, ;;/). And 

 just before closure of the blastopore minute cells with scarcely 

 any cytoplasm are seen budding off from the ectoderm in this 

 region, (Figs. 40, 49, -i'). There is a slight probability that 

 some of the cells in arise by growth of the cells x which would 

 be a case of muscle cells arising from ectoderm as in ccelenterata, 

 etc. But small cells with hardly any cytoplasm are found in the 

 yolk at many stages of the embryo (Figs. 44 and 47, .r) and 

 although I have not closely traced them from cells like x in Fig. 

 40, I think their resemblance in structure indicates a likeness in 

 origin. I think the evidence indicates that all the mesoblast 

 arises from cells turned in from the lip of the blastopore, as is 

 the case in other copepoda, phyllopoda, decapoda and cirripedia. 



The muscle cells when first elongated push the ectoderm 

 toward the center and mass it in a sort of structure which some- 

 what resembles the "dorsal organ" which disintegrates, and the 

 elements of which wander into the yolk. The muscle cells are 

 thus arranged radially just beneath the extremely thin dorsal 



