ic,6 DISCOVERY REPORTS 



It has already been stated that in the immature condition each ovary is filled with five 

 or six eggs, which are almost of the same size, but of these only one or two eggs reach 

 maturity, the others being probably used up as food by the developing eggs. Each egg is 

 composed of a large cytoplasmic part, which stains very lightly in haematoxylin, and a 

 conspicuous nucleus, which is situated basally. The nucleus is of a clear homogeneous 

 appearance and is spherical or oval in shape. As the present material was not fixed for 

 cytological purposes it is not possible to give details, but Gilchrist (191 7) has observed 

 that there are faint indications of chromatin elements and that a conspicuous deeply 

 staining nucleolus is always present. The protoplasmic part is filled with oval or rounded 

 bodies which ultimately give rise to yolk granules. 



The extrusion of the egg and the mode of sperm transference in Cephalodiscus are not 

 known. Masterman suggested that the eggs are liberated at the death of the mother 

 zooids ; but in C. densus and C. fiimosiis the developing eggs have been found at the 

 bottom of some of the tubes, containing in their upper parts zooids which were pro- 

 bably not dead till the time of fixation. 



DEVELOPMENT 



SEGMENTATION 

 The early development of Cephalodiscus was first successfully worked out by Gilchrist 

 (1917), who was able to study the early stages with artificially reared eggs of C. gilchristi. 

 Segmentation is total and equal up to the four-celled stage ; but the succeeding divisions 

 are unequal, probably owing to the great amount of yolk present in the egg. The four- 

 celled stage gives rise to a six-celled embryo, which now assumes an oval shape and 

 shows the commencement of a blastocoeUc cavity. A perfect blastula is formed in the 

 nine-celled stage. The blastocoel is now a wide space filled with a clear homogeneous 

 substance. As the result of further division the blastocoel becomes wider, and at a stage 

 when the embryo consists of about fourteen cells in section, one of the cells at the 

 posterior end proliferates and the daughter cell thus formed sinks into the blastocoelic 

 cavity. During the subsequent stages the cells become more and more diflferentiated, so 

 that the anterior and posterior ends can be distinguished. The blastomeres, which were 

 till now spherical, become elongated in the anterior end, and this transformation slowly 

 extends towards the posterior end until all the cells assume the same shape. While these 

 changes are taking place in the structure of the embryo, a group of three cells situated at 

 the extreme posterior end continues to proliferate daughter cells into the blastocoel, 

 until the latter becomes completely filled with a solid mass of cells. The elongated cells of 

 the external layer are called columnar cells. These now begin to break up so that cell out- 

 lines are completely obliterated. The breaking up of the cell outlines of the external layer 

 of cells has been attributed to the abundance of yolk. The embryo now measures about 

 0-8 mm. in length and a longitudinal section of it is represented in Plate XLIII, fig. i. 

 The external layer is formed of deeply staining protoplasm, which carries numerous rod- 

 shaped bodies, the yolk columns, which are represented by the dark thick lines in the 

 figure. These are formed by the close juxtaposition of each of the multiplying nuclei with 



