EMBRYOLOGY OF THALASSEMA MELLITA 209 



the second and third quartets of these quadrants enter into the 

 formation of the stomodaeum. 



6. Differentiations of the Ectoderm. 



About twelve hours after fertilization, glands and green pig- 

 ment (Text-Fig. 8, B) begin to appear in the ectoderm. The 

 pigment is first noticeable just above the prototroch, arising in 

 the cytoplasm of the cells in close proximity to the nucleus. 

 The glands, on the other hand, first appear in the larger cells 

 between the apical plate and the mouth (Text-Fig. 6, B^. A 

 homogeneous liquid substance is deposited in the outer part of 

 the cell which rapidly increases in amount until it almost en- 

 tirely fills the cell and shoves the nucleus to one side (Text- 

 Fig.. 6, C). This substance does not stain in preserved material 

 in haematoxylin as do mucous glands in general {AnipJiitritc^^ 

 but it does stain brilliantly with Bordeaux red. It would seem, 

 accordingly, that these glands may be of the same nature as the 

 problematic bodies in AnipJiitrite. In the living trochophore a 

 chemical stimulus causes a discharge of a gelatinous substance, 

 as has been noted by Conn. Hatschek ('8i) describes similar 

 glands in EcJiiurus and says that they arise from vacuoles in 

 single cells. Eisig ('98) thinks that they may be excretory 

 and take the place of primary nephridia. This is probably not 

 the case in TJialassema. 



The cells which constitute the ventral neural ciliated region 

 are differentiated early (about fourteen hours) and separately. 

 Later they elongate somewhat and dovetail with one another 

 (Text-Fig. 7, A\ so that in the fully formed trochophore there 

 are two rows of these cells. They are from the first easily dis- 

 tinguishable by their clear refractive protoplasm and larger 

 size. Just below the mouth region these two rows separate 

 and extend on each side of the oral opening to the prototroch. 

 Their method of origin explains the fact that the neural cilia 

 arise in separated bunches and also the manner by which these 

 cilia become connected with those of the prototroch. The 

 apical plate is, in all probability, formed entirely by the rosette 

 cells which, in the trochophore, become elongated and columnar. 



