34 



EMBRYOLOGY. 



The following four diagrams will convey a clearer idea than mere description of the 

 probable stages of this process : 



Fig. 5. 



1. 2. 3. 4. 



Diagrams representing the chorization process in the development of the embryo of Alcyonella. The stage 

 represented in 3 has not been actually seen, but seems to be the only one that will supply the 

 missing link. 



Towards the opening which leads from without into the central cavity the chorization is 

 incomplete, the membrane as it separates being here still held to the walls of the cavity by 

 irregular transverse bands ; these bands check the entire evagination of the membrane, but 

 after a time they disappear, and then the unlining and evagination are perfect. In the interior 

 of the protrusible portion, and before the disappearance of the transverse bands, a polypide is 

 developed (PI. XI, fig. 30). This appears to take place in a manner quite similar to that by 

 which new polypides are produced by gemmation from the walls of the endocystal cavity in 

 the adult. The gemmation of the first polypide is immediately followed by that of another 

 close beside it (PI. XI, fig. 31), so that the young polyzoon has now the appearance of a trans- 

 parent, closed sac, filled with fluid, the posterior part ciliated, the anterior destitute of cilia 

 and partially or entirely pushed back into the posterior by a process of invagination ; while 

 the sac carries within it two young polypides, which are suspended from the inner surface of 

 the unciliated portion. The arms of the lophophore in the young polypides are at first but 

 slightly developed, and there is as yet no trace of tentacula (PI. XI, fig. 30°) ; these soon 

 present themselves in the form of minute tubercles, at first confined to the body of the 

 lophophore (PI. XI, fig. 30'), and then extending along the arms, which at the same time 

 acquire increased proportional length ; the tentacula gradually elongate themselves and acquire 

 cilia. At the same time, the alimentary canal, represented at first by a single small cavity, 

 hollowed out in the body of the polypide, is undergoing development, and oesophagus, stomach, 

 and intestine begin to be distinguishable. The great retractor muscles have become evident, 

 and the funiculus maybe seen extending from the base of the polypide to the walls of the sac 

 'PI. XI, fig. 31). The polypides have, at first, no communication with the exterior, but at 

 an early stage the tentacular sheath, with the parieto-vaginal bands, had become evident, and 

 the fluid in which the embryo floats within the walls of the ovum is soon afterwards admitted 

 to the lophophore of the young polypides. The parieto-vaginal bands would seem to be 

 drawn out by a process of separation from the walls similar to that just described. 



The embryo is still contained within the external membrane of the ovum, which, 

 however, has become much distended, in accordance with the increasing size of the included 

 parts, and at length, giving way, allows the embryo to escape. 



The free embryo (PI. XI, figs. 32, 33, 33", 33*) now swims actively through the surrounding 



