202 DISCOVERY REPORTS 



DEVELOPMENT OF THE BUD 



As the embryonic development of Cephalodiscus is very different from that of the buds 

 an interesting study is afforded by a comparison of the two. The buds are formed from 

 the sucker-hke proximal end of the stalk on either side of the median line. In its earliest 

 recognizable condition the bud is a spherical vesicle which originates as a diverticulum 

 from the interior of the parent stalk. The trunk coelom, which is continued into the 

 stolon, is completely filled with muscle fibres and connective tissue, and the rudiment 

 of the young bud is completely packed with this tissue. This tissue, which is mesodermal 

 in origin, soon arranges itself in such a way as to give rise to a cavity in the middle, which 

 communicates with the trunk coelom in the stolon. The bud emerges from the surface 

 of the stolon as a small knob which rapidly increases in length. The single cavity inside 

 it divides into two by the development of a median mesentery, and later a small spherical 

 vesicle is cut off from the distal end of one of the cavities : this has been regarded as the 

 pericardial sac. The proboscis cavity is believed to be cut off in a similar way from 

 the tip of the other cavity. The two posterior cavities, which are in communication 

 with the cavity of the stolon, give rise at a later stage to the paired trunk and collar 

 cavities. 



As the bud continues to grow, it becomes slightly thicker at its distal end and the stalk 

 by which it is attached to the stolon continues to increase in length. The proboscis now 

 appears at the distal end of the bud. At first it is a small bulb-like structure marked off 

 from the rest of the body by a small constriction (Plate XLIV, fig. 5). It develops very 

 rapidly and assumes the flattened disc-like structure of the adult. The red line appears 

 on its ventral wall and the posterior lobe formed by the two layers of ectoderm without 

 an extension of the coelom is also differentiated at an early stage (Plate XLIV, fig. 4, rl 

 and pi). From the dorsal side of the collar region the arms begin to develop, and by this 

 process when all the parts of the body are unfolded the bud breaks oflt from the parent 

 stolon and becomes an independent zooid. 



The number of buds produced by each zooid varies in the diflFerent species. In 

 C. densus and C. fumosus ten to fifteen buds are found, whereas in C. keftrpi two to five 

 seem to be the maximum number (John, 1931, pi. xxxv, figs, i, 2, 3). 



According to Harmer (1905) and Masterman (1900) the ahmentary canal in the bud 

 makes its appearance as an ectodermal invagination, the orifice of which persists as the 

 mouth ; but the sections which I have obtained are not in agreement with this view. 

 Plate XLIV, fig. 4, is a highly magnified part of a sagittal section of a young bud of C. 

 ?iigresce?ts. The trunk is in the form of a narrow tube only slightly thicker than the stalk. 

 The body wall is filled with aggregations of pigment granules, which give the characteristic 

 colour of the species. The proboscis is very large and the anterior part of its ventral wall 

 is thick and glandular. The arms and mouth are not yet formed, but the alimentary canal 

 is represented by a straight tube which extends downwards from the base of the heart. 

 Below the posterior end of the primitive gut there is an irregular mass of protoplasm, 

 which fills the greater part of the right and left trunk cavities. This is probably derived 



