1904.] NATURAL SCIENCES OF PHILADELPHIA. 385 



the original archenteric invagination (oB, ob, 4C, 5C, 5c, 4D, 5A) 

 while dorsally and anteriorly are seen more yolk-ladened elements 

 whose origin may be traced to the large entoderm cell 4b- and those 

 around it. The stomodaeal invagination breaks through at a much later 

 period between the descendants of 5a and 4b and their neighboring 

 cells, which have been turned in an anterior direction, while doubtless 

 cells from 4c and 5b also push in upon this region with the closure of 

 the blastopore. By the torsion which the enteron has undergone the 

 upper mass of large yolk-ladened cells is moved more and more to the 

 left, while in like manner 4a- turns to the right. While this is occurring 

 the invaginating sliell-gland has pushed the anterior and posterior 

 walls of the enteron very closely together, both enteric and cleavage 

 cavities being practically obliterated (fig. 86). When this structure 

 evaginates the enteron again opens out and has then lost its elongated 

 form, being rounded with its wall cells in close contact (fig. 87). 



In Umbrella as well as in Fiona 4b' occupies the anterior end of the 

 enteric mass pushing up into the pointed apex of the gastrula, and the 

 same is true of Aplysia in which there are but two large blastomeres, 

 though according to Blochmann's nomenclature such does not appear 

 to be the case. In later stages the positions of the large cells of the 

 fourth quartet of Umbrella and Fiona are identical. The intestine of 

 Umbrella is said to be formed by C" and D" (5c and 5d), which, as Hey- 

 mons did not take into consideration an entoblastic contribution from 

 4d, correspond fairly well to the conditions foimd in Fiona, where these 

 cells lie just at the place of origin of the intestine and may well take 

 part in its future development. The cell-lineage of the archenteron 

 of Crepidula is given as follows: "The four macromeres form the roof 

 of the archenteric cavity. The cells of the fifth quartet form its lateral 

 boundaries, arching the cavity on all sides save the posterior. Here 

 the archenteric cavity runs backward between the cells 5C and 5D (5c 

 and 5d) nearly to the posterior boundary of the egg. The cells of the 

 fourth quartet come together on the ventral side of the archenteron, 

 forming its floor anteriorly and ultimately giving rise to some of the 

 many small cells which form that part of the mesenteron, adjoining 

 the stomodseum." The intestine arises from the posterior lower right 

 region of the enteron as a tube-like evagination, formed from the entero- 

 blasts derived from 4d and neighboring small endodermal cells and 

 ending blindly against the ectoderm. Later it elongates and the end 

 is carried somewhat upward along the right side by trosion of the larva. 

 It contains a lumen from the first. As the stomach begins to enlarge 

 it is seen to be bounded by large cells dorsally and anteriorly in its lower 

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