28 ORGANIZATION AND CELL-LINEAGE OF ASCID1AN EGG. 



(4) In the 16-cell stage the 8 dorsal endodermal cells are yellow and have 

 small nuclei ; the 8 ventral ectodermal ones are clear (p. 50). 



In the identification of individual cells and their axial relations Seeliger was 

 much at fault. The small cells of the 4-cell and later stages are certainly not ante- 

 rior in position but posterior, as has been shown by Van Beneden and Julin, Cha- 

 hry. Samassa, and Castle; while the two larger cells of the 8-cell stage are not ven- 

 tral but dorsal in position, not posterior hut anterior, as their relations to the two 

 small posterior cells show. Seeliger therefore mistook anterior for posterior, dorsal 

 tor ventral and consequently right for left; in short, he committed all the mistakes 

 possible in orientation. 



3. Samassd's System. 



Ten years after the publication of Van Beneden and Julin' s work, Samassa 

 (1894) working on Ciona and Clavellina reached very different conclusions from 

 those set forth by the first named authors. With the first four conclusions of Van 

 Beneden and Julin mentioned above he agrees, save that in the unsegmented egg 

 he claims that only the median plane and the anterior and posterior, but not the 

 dorsal and ventral, poles can he recognized. With regard to the identification of 

 the dorsal and ventral sides he held that Van Beneden and Julin were completely 

 in error and that they had mistaken the dorsal for the ventral, the endodermal for 

 the ectodermal pole in all stages up to the 44-cell stage. As the most important 

 evidence of this false orientation Samassa cites Van Beneden and Julin' s figures '.' c 

 and 10 c, which represent optical sections in the sagittal plane of a 32-cell and a 

 44-cell stage respectively. In the first of these the ectoderm cells are shown as 

 columnar, the endoderm cells as flattened; whereas in the second, figure 10c, the 

 ectoderm cells are flattened and the endoderm columnar. '"The figures of these 

 two authors." says Samassa, "are sufficient to show that figure 10c is properly and 

 figure c falsely oriented ; in both cases the cylindrical cells belong to the endoderm 

 and are dorsal in position." The words of Samassa directed against Van Beneden 

 and .Julin apply with equal or even greater force to himself : "Van Beneden and 

 Julin have not once sought," he says, "to bring forward one fact in support of this 

 remarkable transformation." With the exception of the worthless a priori argu- 

 ment that cells which have once been cylindrical must always remain so Samassa 

 has not produced a single argument or fact in favor of his contention. 



4. Castle's System. 



In tin- same year Castle (1894), in a preliminary paper and again in his 

 final paper (1896) on the early embryology of Ciona intestinalis, reversed the orien- 

 tation maintained by Van Beneden and Julin and held with Samassa that in all 

 stages preceding the 44-cell stage the Belgian investigators had mistaken dorsal 

 for ventral and vice versa. Furthermore, after having studied the formation of 

 the polar bodies, he was lead to the truly remarkable conclusion that these bodies 

 in ascidians are formed at the endodermal pole, whereas in all other animals, so 



