ON THE DEVELOPMENT OF 

 CEPHALODISCUS 



By C. C. John, M.A., d.sc, d.i.c. 

 (Plates XLIII, XLIV) 



INTRODUCTION 



THE observations which have hitherto been made on the development of Cephalo- 

 discus are mostly of a fragmentary nature owing to the difficulty of obtaining the 

 eggs and larvae. From the material procured by the Challenger expedition, Masterman 

 (1900) recorded a few segmenting eggs. Andersson (1903), in a preliminary note, showed 

 that the segmented embryos left the parent colony in the form of ciliated planulae. 

 Harmer (1905) observed that the eggs were heavily laden with yolk, that their segmenta- 

 tion was holoblastic and nearly equal, and suggested that they might give rise to solid 

 embryos in which the endoderm arises by delamination. Further, he showed that the 

 five body cavities of the adult arose at an early stage in development. Andersson (1907), 

 in his account of Cephalodisciis collected by the Swedish South Polar Expedition, de- 

 scribed a few more stages. He stated that there was a gastrula-like stage, in which there is 

 a centrally placed mass of yolk with a narrow lumen. He believed that this central mass 

 of yolk represented the endoderm, which originated by a process of invagination. He also 

 confirmed the observations of Harmer regarding the early appearance of the body 

 cavities. SchepotiefT (1909) also described a few stages, but his account added nothing 

 to the elucidation of the gastrula stage described by Andersson. He confirmed the exist- 

 ence of the planula-like larva, and described a later stage which was also free-swimming 

 and in which the five body cavities were visible. He regarded the central mass of yolk 

 in the embryo and the larva as representing the endoderm formed by invagination. The 

 account of Braem (191 1) does not add anything further to our knowledge of the develop- 

 ment of the group, but he drew attention to the resemblance between the free-swimming 

 larva of Cephalodisciis and that of certain Polyzoa. 



The first connected account of the development was given by Gilchrist (1917). He 

 collected the eggs and larvae of C. gilchristi from the coast of South Africa and at- 

 tempted to rear them artificially. During the first year of his work he was able to obtain 

 all the early stages from the beginning of segmentation up to the free-swimming larval 

 stage ; but during the second year, when he repeated the trawling operations, he was not 

 able to get even the adult colonies, so that further observations were rendered im- 

 possible. He described the segmentation, the origin of yolk, endoderm, the coelomic 

 cavities, the pigment granules and the structure of the free-swimming larvae. Though 

 some of his observations are open to doubt, his account of the early segmentation is a 

 very valuable help for all further research. 



