28 Edmund B. Jfilscn. 



arated, both continue to segment for a time as if still forming 

 part of an entire embryo, the second and third polar lobes form- 

 ing in normal fashion in the CD half; but in the end both com- 

 pletely close, gastrulate, and form activ^ely swimming larvae. The 

 two larvae agree in possessing a closed, though often somewhat 

 asymmetrical or confused prototroch, but otherwise show the fol- 

 lowing characteristic and constant differences. The AB (smaller) 

 larva, closely resembles, except in size, that derived from an 

 entire egg from which the polar lobe has been removed, invaria- 

 bly lacking a post-trochal region and apical organ (Fig. 46). 

 The CD (larger) larva, on the other hand, possesses both these 

 structures, both of which may be as large as in a whole embryo 

 (Figs. 42-45). These larvae vary greatly in form, but in gen- 

 eral are asymmetrical and, as may be seen by a comparison of 

 Figs. 45 and 29, possess a post-trochal region that is almost in- 

 variably relatively too large, and a pre-trochal region relatively 

 too small as compared with a normal larva. As in the AB half, 

 the prototrochal cilia frequently show a confused arrangement, 

 the regular rings of the normal larva being more or less broken 

 up. In like manner, if the four blastomeres of the 4-cell stage 

 be isolated, only the larva from the D (largest) quadrant develops 

 these two structures (Fig. 47), while those from A, B or C are 

 nearly like those derived from the AB half, though only half as 

 large (Figs. 48, 51). Like the CD ^-larvae the D ^^-forms are 

 variable in form; but whenever they complete what may be con- 

 sidered their normal development they show the post-trochal region 

 very much too large, and the pre-trochal region much too small 



(Fig. 47)- 



All these larvae show a very high mortalit}^ but I have kept the 

 ^-larvae as late as the beginning of the fourth day ( Fig. 51), and 

 the 54-larvae nearly as long. The smaller larvae (the AB half, 

 or the small quarters) show a greater tenacity of life, swim more 

 actively, and become less irregular than the larger ones. In the 

 end, however, all the forms become irregular and finally wholly 

 disintegrate, without producing normally formed trochophores or 

 regenerating the missing structures. The CD ^-larvae of 24 

 hours sometimes approach the form of normal larvae of the same 

 age, though always showing the false proportions of the pre-tro- 



