MONOGRAPH OF THE EXISTING CRINOIDS. 377 



tube are entirely solid, consisting of large egg cells which show germinal vesicles. 

 Here and there on the periphery lie small, younger egg cells, while undeveloped 

 cells, possibly primitive germ cells which have failed to mature, lie between them. 

 An outer epithelium composed of flattened cells which ensheathes the entire 

 solid cylindrical ovarial structure can always be made out. 



Prof. Gilbert C. Chubb has made a detailed study of the growth of the 

 oocyte in Antedon bifida, his material having been obtained at Plymouth, England. 

 The following account is taken from his memoir on the subject published in 1906 : 



In the maturing ovary the growing oocytes form a more or less solid mass 

 of cells between which oogonia are still present in considerable numbers. The 

 only constant feature in the arrangement of the germ cells consists in the in- 

 variably peripheral position of the still growing oocyte. As there is no germinal 

 epithelium, the new generations of oocytes being derived from the scattered oogonia, 

 there must either be an active peripheral migration of the oocyte prior to the 

 commencement of its growth, or else only those oocytes which happen already 

 to possess this position ever commence the latter process. The fact that this 

 position should be maintained during the "gi-owth " period of the egg, and that 

 it should be frequently'' lost while yolk formation is still in progress, is of great 

 interest in connection with the question of the relation of yolk formation; that is, 

 the actual appearance of the definitive yolk spherules, to the general cell metabo- 

 lism. In the moi-e mature individuals the scattered oogonia are only rarely found 

 in mitosis, while in the mature pinnules these abortive oogonia have undergone 

 considerable degenerative changes. 



When freely floating the oogonia are amoeboid in form, but when closely 

 packed their outline is not always distinguishable. The nucleus is approximately 

 spherical and measures from 3 [j. to 5 ix in diameter. There is a well-marked nuclear 

 membrane, and the chromatin is in the form of irregular and deeply staining 

 masses at the nodal points of a clearly defined linin reticulum. In many nuclei 

 the meshes of this reticulum are so large that the greater part of the chromatin 

 forms a series of peripheral masses on the nuclear membrane, while the remainder 

 is massed around the nucleolus, the linin being reduced to a few strands radiating 

 out from the surface of the nucleolus to the peripherally placed chromatin masses. 

 There is a single nucleolus ranging from 1 [x to nearly 2 ix in size and apparently 

 always peripheral in position. It appears as a pale brown or yellow perfectly 

 regular and sharply defined spherule surrounded by the deeply staining chromatin. 

 In some cases, especially in the very immature pinnules, there are not only two 

 nucleoli, but the nucleus itself is distinctly double. 



In the dividing oogonia the nucleus measures 5 [a to 6 [a and the cell body 7 |a to 

 8 [J., thus showing a considerable increase in size over that of the resting oogonia. 

 A split spireme thread is formed, and this enters a synapsis stage, becoming 

 aggregated around the nucleolus on one side of the nucleus. During this process 

 the thread shortens and thickens, so that the longitudinal split usually entirely dis- 



