vol.3.] Robertson. — Embryonic Fission in Crisia. 127 



arrange themselves in parallel lines forming the incipient tenta- 

 cles {ml. bd. 1 and 3 in. tent.), while those in the lower portion 

 form into a hollow sphere to produce the cavity of the stomach 

 {St.). The proximal bud of PI. XIII. fig. IT. {pd. bd. l) , and 

 the anterior bud of Fig. 16 [pil. lid.), represent a stage in the 

 development of polypides identical with those in Fig. 11. The 

 significance of the union of ovum and polypide in these two 

 cases is further revealed by the polypide just proximal to the 

 young bud (PI. XIII, Fig. 1G, pd. 2). Here attached to the caeca! 

 end of the stomach of an adult polypide, is a veritable embryo 

 (imh.) consisting of at least three cells. That these are blasto- 

 meres of an embryo, and not merely a bunch of ova, is shown by 

 i lie condition of the nuclei. The two upper cells have apparently 

 just completed their mitosis, and the nuclei are relatively small. 

 The nucleus of the lower cell has lost its nuclear wall, and the 

 cell is preparing for division. This case affords an explanation 

 of those instances where an ovum is held by a delicate membrane 

 at the proximal end of a column of cells, and represents a kind 

 of union that may occur between a bud and an egg, but one in 

 which it<> uric, II result*. The next older polypide (pil . j) pos- 

 sesses neither ovum nor embryo. Young embryos of two or 

 three cells are not uncommon upon buds or young polypides near 

 the growing points, although single ova attached to young buds 

 and to adult polypides are of more frequent occurrence. This 

 figure (Fig. 16) represents a typical section through the growing 

 tip of a young colony. In the growing tissue, right and left, 

 ova are more or less numerous. Proximal to this, the youngest 

 bud frequently possesses an ovum, and below this, one or two 

 polypides may carry a single ovum each, or a young embryo. The 

 coexistence of polypide and embryo or ovum has not been pre- 

 viously noted in this subclass of bryozoa, and while it is probably 

 an abnormal condition for Crisia, it is, perhaps, indicative of 

 a more primitive method of reproduction. I have never observed 

 this except at the height of the breeding season, when ova 

 are being rapidly produced. In the older portions of the 

 colony neither eggs nor embryos have been found, nor have 

 larva? been obtained, in any of the older zooeeia. These embryos, 

 apparently, never attain complete development, but are absorbed. 



