PHALLUSIA MAMILLATA 397 



constitution and destiny; the ectoderm, endoderm, anterior 

 mesenchyme, caudal mesenchyme, muscle cells, chorda ring and 

 neural ring of the gastrula are composed of wholly similar cells 

 in all of these genera. 



The elongated gastrula of Phallusia is almost identically like 

 that of Cynthia and Ciona in the number, relative sizes and his- 

 togenetic character of its constituent cells, and in the cell lineage 

 of its primitive organs, such as the neural and chordal plates, 

 muscle and mesenchyme areas, ectoderm and endoderm. 



The larva of Phallusia closely resembles that of Cynthia and 

 Ciona, but all the larvae of the former hatch, whereas a varying 

 number in the latter genera undergo metamorphosis within the 

 egg chorion. Not infrequently slight abnormalities appear 

 among otherwise normal larvae ; the most usual form of abnormal- 

 ity is that in which the neural plate does not infold and the eye 

 pigmerlt is more or less scattered in surface cells; in other cases 

 the notochord may show sharp bends and swellings. Such ab- 

 normal larvae frequently hatch from the egg membranes. 



POTENCY OF SINGLE BLASTOMERES OF THE EGG OF PHALLUSIA 



Certain blastomeres of the 2-cell, 4-cell and 8-cell stages of 

 Phallusia were killed or injured by shaking the eggs vigorously 

 in a vial in the manner described by Driesch (1895). In general, 

 however, I found it easier and more effective to cause these 

 injuries to single blastomeres by spurting the eggs from a pipette 

 into a watch glass. The eggs were drawn into a pipette having 

 a moderately fine point and were then spurted out with con- 

 siderable force. The eggs of Phallusia are so delicate that most of 

 the eggs treated in this way are injured in whole or in part; 

 some of them do not develop at all, in other eggs one or more of 

 the blastomeres may be injured so that they do not develop, 

 while other blastomeres remain capable of development. Usually 

 the injured blastomeres become more opaque than the uninjured 

 ones, so that it is possible to isolate at once partial eggs in which 

 one or more of the blastomeres have been killed. 



