ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 875 



clefts in Vertebrata, and therefore the Aseidians are " Chordata with 

 a single pair of branchial clefts while the Vertebrata are furnished 

 with several and the Cephalochorda with a great number." The 

 stigmata of the adult Aseidians clearly do not correspond to the 

 branchial clefts of the Vertebrata and Appendicular ia, but are 

 secondary structures. 



Segmentation of Simple Aseidians.* — L. Chabry's paper is 

 supplementary to the recent important memoir of Van Beneden and 

 Julin. His object is to show that the segmentation of Aseidians 

 really differs but little from the regular mode, and that it is possible 

 to approximate them to one another. 



He describes the stages in which there are 2, 4, 8, 16, 22, and 30 

 cells; the planes parallel to the equator ("tropical planes") which, 

 in a case of regular segmentation, lead to the succession of the 1 6- by 

 the 32-stage are, in Aseidians, broken up into small parts, which do 

 not regularly follow one another. In other words, the planes are 

 more or less distant from the poles, and so give to the surface of the 

 sphere an irregular appearance, which can only be brought into 

 relation with that of regular segmentation by attentive study. The 

 author enters into an account of his views on the subject, and 

 expresses his belief that he has demonstrated the existence of a true 

 comparative morphology of segmentation, the aim of which is to relate 

 to one another the different modes of division of the yolk, and to find traits 

 common to a number of animals in the midst of apparent irregularities. 



Development of Social Ascidians.t — 0. Seeliger commences with 

 an account of the mode of cleavage in Clavelina, as he was not able 

 to observe the process of fecundation, or the formation of the polar 

 cells. The cleavage is not so regular as is ordinarily supposed, and 

 it is interesting to note how the cleavage and the formation of the. 

 two primary germinal layers are combined in one process. Bilateral 

 symmetry is at once observable, and a distinction into fore and hind 

 ends appears with the 4-stage; the back and ventral aspect are 

 seen in the 8-stage, when, too, the two primary layers begin to be 

 differentiated. 



Gastrulation commences with the appearance of 16 cells- the 

 author does not feel able to speak definitely as to the mode of origin 

 of the notochord ; the blastopore appears to close up in much the 

 same way as ia AmjyhioxuK, and the author suggests that the unaltered 

 condition of the lower edge is due to the first appearance of the nerve- 

 tube at this point. As the hinder portion of the body begins to in- 

 crease in length, the embryo takes on a pyriform aspect, and it is in 

 this region that the greatest activity in developmental processes ia 

 now apparent. The first organ to appear is the notochord, and its 

 mode of formation is seen to be intimately connected with the mode 

 of closure of the blastopore ; just as in Amphioxus the cord extends 

 from before backwards, and the fact that the hindermost cells are the 

 first to become apparent, is due to the form of the blastopore. The 



• Journ. Anat. et Pliysiol. (Robin), xx. (1884) pp. 387-92. 

 t JenHiuoli. ZcitHclir., xviii. (1881) j)p. 45-120 (8 pis.). 



3 M 2 



