ORGANIZATION AND CELL-LINEAGE OF A.SCIDIAN EGG. 7 



In the face of all this antecedent work, it may seem superfluous to devote vet 

 another paper to this subject, and nothing was further from my purpose when I 



began. It was in the attempt to find out the manner in which the ascidian < 



conies to form its polar bodies at the endodermal pole, as described by Castle, that I 

 was led to conclusions radically different from his, as will be described later, and this 

 induced me to make a detailed study of the cell-lineage of three different genera of 

 simple ascidians. In such a Held it may lie expecting too much to hope that m\ 

 observations will meet with general acceptance; hut perhaps it may he proper for 

 me to say that I have spared no pains or labor to make them accurate. 



('. Material and Methods. Early in July. 1903, while working at the Marine 

 Biological Laboratory, Woods Holl, Mass.. I began the study of the maturation 

 and fertilization of the egg of Ciona intestinalis (L.) Flemming, with the aim men- 

 tioned in the preceding paragraph. Only a small number of these animals was to 

 be found at that time at Woods Holl. though they occurred more abundantly later 

 in the summer. I therefore turned my attention to two other simple ascidians. 

 Molgula manhattensis Verrill and Cynthia (Styeld) partita Stimpson, both of which 

 occur in considerable numbers in the Woods Holl region. The verj first lot of the 

 living eggs of Cynthia which I examined showed a most remarkable phenomenon 

 and one which modified the whole course and purpose of my work ; for there on 

 many of the unsegmented eggs, which were of a slate-gray color, was a brilliant 

 orange-yellow spot, which in other eggs appeared in the form of a crescent or band. 

 Further observation showed that this crescent became divided into two equal parts 

 at the first cleavage and that it could be followed through the later cleavages and 

 even into the tadpole stage. I thereafter, for a considerable portion of the summer, 

 devoted myself to the study of the living eggs of Cynthia, and a record of these 

 observations will be found in the body of this work and in plates I-V. Afterward 

 I took up also the living eggs of Ciona and Molgula, and finally I fixed and pre- 

 pared for microscopical examination, both as whole objects and as serial sections, 

 the eggs and embryos of all three of these genera. 



Castle (1896) has described in considerable detail the time and manner of egg 

 laying in these three genera, and his observations lean entirely confirm. The eggs 

 of Ciona and Molgula are laid in the early morning, a little before daybreak, while 

 those of Cynthia are laid in the late afternoon, a little before sunset. These 

 ascidians rarely lay eggs the first da\ they are in the laboratory. Since the 

 yellow pigment of the egg of Cynthia is difficult to observe by artificial light, it 

 was necessary to take eggs from the ovary or oviducts and artifically fertilize 

 them in the morning in order to lie able to study by daj light the later stages in the 

 development. A lame proportion of such eggs never develop, though the eggs 

 seem ripe and the spermatozoa are active; however, some of them develop into 

 normal embryos and tadpoles, and from such I have obtained material for the study 

 of the later stages of the living egg. Whenever possible, however. 1 have relied 

 upon eggs which were normally laid and fertilized, inasmuch as all such develop 

 normally. In Ciona and Molgula it is very eas} to artificial!} fertilize the eggs; 



