G ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



that they can be homologized with one another. May not these differentiations 

 appear at still earlier stages and " organ-forming germ regions" be marked out in 

 the egg before cleavage begins'.' Finally does the organization of the egg arise 

 de novo in the ovary, or ma\ a certain part of it be carried over from generation to 

 generation, and is this early organization of the egg in any way different from the 

 organization of any cell? These are problems of profound importance which lie at 

 the basis of any thorough study of development, inheritance and evolution. 



B. Ascidian Embryology. Anyone who has observed the ascidian egg will 

 understand why it has been such a favorite object of study. The cleavage of the 

 egg is so beautifully regular and can be observed so readily in life that it is not 

 surprising that ascidians were among the first animals to which the "cell-lineage" 

 method was applied. It is surprising, however, to find such diversity of opinion 

 with regard to the development of these animals ; even in some of the most import- 

 ant points in the early development, such as the relation of the poles of the egg to 

 those of the gastrula, or the cell origin of the germ layers, scarcely two authors 

 agree, in spite of the fact that these eggs are perhaps as favorable as any others in 

 the whole animal kingdom for the stud}' of these problems. Under the circum- 

 stances it is impossible to avoid the conclusion that a large amount of very careless 

 work has been done in this field. Accuracy is ever the avowed aim of science, and 

 when one finds scientific work unpardonably inaccurate he may justly feel indig- 

 nant. For some reason or other ascidian embryology has ever been a field of con- 

 flict and confusion. In some cases controversy has raged for years around a mere 

 blunder which has thus gained a sort of immortality rarely attained by accurate 

 work . 



But ascidian embryology furnishes illustrations not only of inaccurate work; 

 it also affords some of the most classical examples of accurate and substantial 

 research. Considering the time when the}- were written, Kowalevsky's papers 

 (1866, 1871) are models of accuracy. The beautiful studies of Van Beneden and 

 Julin on the segmentation of the ascidian egg and on the morphology of the tuni- 

 cates (1884, 1886) surpass in excellence anything which had been done up to that 

 time, and in some respects they have not been equalled by an}' more recent work 

 on the development of these animals. Chabry's (1887) classical paper on the nor- 

 mal and teratological embryology of ascidians is a masterpiece, one of the first 

 and best illustrations of the application of the experimental method to the study 

 of embryology. After this, the most important work dealing with the early devel- 

 opment of ascidians is that of Castle (1894, 1896), although it is marred by some 

 fundamental mistakes; he has applied in detail the method of cell lineage to the 

 study of the development of Ciona intestinalis, and has followed the history of 

 the individual cleavage cells farther than had any of his predecessors. Other work 

 which deserves mention has been done by Kupffer (1870), Seeliger (1885), Davidoff 

 (1889), Samassa (1894) and others, and will be referred to later in the body of 

 this work. 



