740 SUMMARY OP CURRENT RESEARCHES RELATING TO 



There are other phenomena which are not purely physical, but are of 

 a vital nature ; these are the amoeboid deformations. M. Chabry has found 

 that these do not take place by chance, but that certain deformations 

 are repeated in exactly the same way for every cell in the normal egg. 

 Each blastomere is, therefore, characterized, when in the normal state, not 

 only by size, form, and proper position, but by a succession of determinate 

 forms. 



From the teratological point of view, it is of interest to note that 

 injury to a blastomere results in the suppression of development of the 

 organs potentially contained in that cell ; but at the same time it is to 

 be noted, that it occasionally happens that by the death of one cell the power 

 of the survivors is changed, and they then give rise to parts which, in 

 normal circumstances, they would not have produced. 



jS. Polyzoa. 



Development of Cyclostomatous Marine Bryozoa.* — Herr A. Ostroumoff 

 did not succeed in finding any earlier stage than one which consisted of 

 epiblast and endoblast, such as has been figured by Metschnikoff in cross 

 sections ; in longitudinal sections the author found at the equatorial plane, 

 on either side of the embryo, three large epiblast cells ; these correspond to 

 the corona or ciliated zone which exists in the larvse of other marine 

 Ectoprocta, where it consists of a row of long cells. The embryo grows 

 rapidly; when the cells of the vegetative pole invaginate to form the 

 sucker cavity the embryo has a remarkable resemblance to a gastrula. 

 Later on the circular fold which is to form the mantle appears round the 

 animal pole ; its cavity is not formed by the insinking of this pole, but in 

 consequence of the growth of the margin of the fold. With regard to the 

 share taken by the endoderm, Barrois and Metschnikoff differ ; the author 

 detected a small endodermal cavity, the wall of which was formed of small 

 flattened cells, but at the sides the walls were seen to be breaking up. At 

 the end of the period of embryonic development there is no sign of this 

 cavity. Herr Ostroumoff thinks that Metschnikoff took the mantle-cavity 

 for the endodermal one. 



The larva, after swimming about freely for some time, attaches itself 

 by its sucker, the hind wall of which is extended into a broad round basal 

 plate, which secretes largely a cuticular substance on its free surface. 

 The mantle bends down and fuses with the periphery of the basal plate, 

 the fusion occurring in the part where are the higher, apparently glandular 

 cells. The ring-like vestibulum becomes completely closed, the cilia soon 

 cease to move, and the cells that bear them fall away. At one point the 

 ectodermal cells become differentiated in the form of a cellular plate 

 which becomes more distinct a little later on ; this is the ectodermal 

 rudiment of the polypide. The period at which the plate becomes marked 

 off varies with various Cyclostomata. 



A striking point is the development of a special endothelial sheath 

 around the nutrieut tube ; this is a temporary structure, which, after a 

 time, completely disappears. 



The stage which the author, with Barrois, looks upon as that of the 

 primitive Bryozoon, presents the following characters ; it consists of two 

 surfaces divided by a cleft which leads into the vestibular ring ; the vestibule 

 is formed by the ciliated cells of the larval skin and the anterior wall of the 

 sucker ; one of its surfaces mny be called the pallial, since it is formed at 

 the expense of the walls of the mantle-cavity, while the other is the basal 



=!■ MT. Zool. Stat. Neape], vii. (1887) pp. 177-90 (1 p].). 



