394 EDWIN G. CONKLIN 



have not been able to recognize specifically differentiated areas 

 in the living egg, by the peculiar texture of the protoplasm, as 

 was true in the case of Ciona. In the main these remarks are 

 true also of the cleavage stages, the gastrula, and the larva of 

 Phallusia; little differentiation of any kind can be seen in the 

 living cells of these stages 



Nevertheless it is quite evident that differentiations of the 

 protoplasm must be present in the larva, at least, with its several 

 organs and tissues, even though they may not be visible in the 

 living condition; and these differentiations may be demonstrated 

 by means of differential staining. The endoderm contains a 

 substance, chiefly yolk, which stains very little if at all in Mayer's 

 haemalum, followed by eosin; the notochord also stains very 

 little; the muscles of the tail are stained red, while the nervous 

 system stains blue. In the gastrula and cleavage stages sub- 

 stances which stain in a similar manner may be recognized in the 

 cells which enter into the formation of these different organs of 

 the larva. And even in the egg immediately before the first 

 cleavage, substances showing these same staining qualities are 

 definitely localized in certain regions of the egg. 



Sections of eggs before the first cleavage show a crescent of 

 non-granular, homogeneous substance around the posterior side 

 of the egg, identical in position with the ''mesodermal crescent" 

 of Ciona and Cynthia. In well stained eggs this crescent is 

 very sharply marked off from the adjoining ooplasm; it contains 

 no yolk and is deeply colored by plasma stains. At this stage 

 there is also a considerable amount of unstained, or faintly 

 stained yolk which is peripheral in position at the time of the 

 first cleavage ; and there is an area of granular protoplasm around 

 the nucleus or mitotic figure, which stains blue in the fluids 

 mentioned above. Each of these three substances is divided 

 bilaterally by the first cleavage plane (photos 1, fig. 2.) 



Immediately after the first cleavage much of the yolk collects 

 at the vegetative pole of the egg, anterior to the mesodermal 

 crescent (photo 2, fig. 1); this is the area of the future endoderm 

 cells. The remainder of the yolk lies at the periphery of the 

 egg and ultimately goes into the ectoderm and chorda. In 



