The Development of Ischnochiton. 577 



cell. The terms right and left are employed in the usual sense, 

 i. e., right is clockwise, left is anti-clockwise. A cleavage is oblique 

 to the right or , following Lillie ('95) dexiotropic, when the 

 upper moiety lies to the right of the lower; it is oblique to the left, 

 or 1 e i o t r o p i c , when the upper moiety lies to the left of the 

 lower." 



I. The Uiisegmciited Ovudi (Fig. 1). 



The ovum of I. magdalenensis varies in color from light pink to 

 pinkish gray. It is perfectly spherical and measures exclusive of the 

 chorion 0,4 mm in diameter. It is densely packed with a finely 

 granular yolk that renders it perfectly opaque, making observations very 

 difficult on the living egg. The protoplasmic portion at the animal pole 

 is very slightly developed and is to be determined by sections only. 

 The cleavage nucleus is slightly eccentric lying on the side toward 

 the animal pole, and is characterized by a finely granular chromatin 

 network suspended in a relatively abundant achromatic substance. This 

 feature of the nucleus is the same for all cells during a considerable 

 period of the ontogeny and often renders it difficult if not impossible 

 to follow some of the phases of development of the entodermal cells, 

 a difficulty that is not lessened by the abundance of yolk. 



Two polar bodies are formed and the chromatin of the first one 

 occasionally redivides, though in no case have I seen the division 

 affect the cytoplasm. The chromatin consists of densely aggregated 

 irregular knots staining intensely, while the cytoplasm is perfectly 

 transparent and is little if at all stained by logwood dyes. In strip- 

 ping the chorion from these eggs the polar bodies are usually dis- 

 lodged, although normally they persist a least until the 160 cell 

 stage. 



Each egg is enclosed in a chorion bristling with many fork-like 

 processes (P'ig. 1) almost exactly identical to those figured by Kowa- 

 LEVSKi for Chiton polii. It is much thinner than the chorion of 

 Mopalia lignosa or Katharina tunicata whose eggs are laid singly, 

 but the albumen in which the eggs are embedded no doubt acts as 

 an additional protective envelope. 



I have taken up the study of the oogenesis only in a general 

 way, but there seems little doubt but that Garnault's observations 

 on the formation of the chorion are correct. This structure is ac- 

 cordingly the metamorphosed follicular epithelium, and not a product 



