322 RESEARCH SEMINAR. 



and lateral sides, and passing then into the oesophagus. The 

 dorsal wall of the pharynx is supplied by a plexus of nerve-trunks 

 from the brain and the nearest portions of the connectives. The 

 plexuses on the two sides of the body are seldom alike. From 

 the posterior lobes of the brain several nerves pass to the nuchal 

 organ. In the caudal region, the number of annuli in each 

 somite, and with them the number of nerves, may vary from 

 three in the anterior part to nine or ten in the posterior part. 



Augtist 14. The Organization and Orientation of the Ascidian 



Egg. By E. G. CONKLIN. 



During this summer I have studied the development of three 

 species of solitary ascidians, viz. Styela {Cynthia) partita, Molgula 

 Manhattensis and dona intcstinalis, with the purpose of finding 

 out how much of organization can be recognized in the unseg- 

 mented eggs of these animals. 



In the living eggs of all these ascidians, but particularly in the 

 first mentioned, one can recognize the substance of the ectoderm, 

 the endoderm and at least a portion of the mesoderm before the 

 first cleavage occurs. 



The spermatozoon enters at the vegetative pole in an area of 

 cytoplasm free from yolk and in Styela a dense mass of orange 

 pigment aggregates at this spot and slowly spreads over the 

 vegetative hemisphere. Subsequently it withdraws to one side 

 of this hemisphere, thus forming an orange crescent which lies 

 just below the equator on the posterior side of the egg. The 

 vegetative pole then becomes slate-gray in color and the animal 

 pole a light gray. The study of the subsequent development 

 shows that all the axes of the future animal are now established 

 and that the slate-gray substance forms endoderm, the light gray 

 ectoderm and the orange crescent the muscular system of the 

 tadpole. In Ciona and Molgula the crescent is present as in 

 Styela, but is nearly colorless, while the substance of the ecto- 

 derm and of the endoderm may also be recognized in the unseg- 

 mented egg. 



An incidental result of this work is to prove beyond question 

 than Van Beneden and Julin were right in their orientation of the 

 ascidian egg and that the polar bodies are found at the ectodermal 

 and not at the endodermal pole, as Castle has maintained. 



