120 BULLETIN OF THE 



mic epithelium of the region from which they arise. It is owing to 

 this method of origin that the epithelium comes to clothe both faces 

 of the partition. The process by which the muscle fibres move into 

 the ccenocool appears to be this. The end of a fibre nearest the roof 

 becomes fixed to a certain part of the muscularis of the roof, and is 

 left behind with it when the margin is carried outward (potentially) as 

 the result of cell proliferation. Thus from a nearly horizontal position 

 the fibres attain a direction at first oblique, and then perpendicular to 

 the sole. In some instances the upper ends of the fibres move through 

 an arc of more than ninety degrees, so that they are ultimately directed 

 upward and inward, i. e. towards the centre of the colony. (Compare 

 mu, mu, mu" 9 Fig. 55, Plate VI.). This process is also indicated in 

 two horizontal sections (Plate X. Figs. 95 and 96), the former being 

 nearer the sole than the latter. This is a region of active budding, and 

 in consequence new compartments or branches are being rapidly formed. 

 The numbers 3, 4, 5, and 6 (Figs. 95, 96) show the positions of young 

 partitions, which are shorter above than below, owing to the oblique 

 position assumed by the innermost muscle fibres of the partition. The 

 oblique position is due to the fact already demonstrated (Fig. 55, 

 Plate VI.), that the tectal end of the muscle of the partition first 

 appears at the margin nearer the sole than the roof. At 2 (Fig. 96) 

 there is apparently an interesting case of the formation of a new par- 

 tition by the detachment of certain fibres from the muscularis of an 

 old one. The fibres, moving away laterally, take with them a covering 

 of ccelomic epithelium. Near the sole this process has progressed far- 

 ther than it has nearer the roof, so that in Fig. 95 the detachment 

 appears complete, whereas in Fig. 96 the union is still visible. This 

 method of formation is intelligible when one considers that the muscu- 

 laris of the partition often contains more than a single layer of muscle 

 fibres. Thus, in Figure 87, mu., there are two or three layers of fibres 

 in the section. Figure 86 represents a section cut vertically and at 

 right angles to a partition near its union with the marginal wall of the 

 colony, and shows three fibres of the longitudinal or inner layer of 

 the muscularis lying side by side in the partition. 



B. — Comparative and Theoretical Review of the Observations on 

 the Origin of the Individual. 



What bearing have the facts here adduced on those given for other 

 groups of Bryozoa, and what is their probable significance in relation 

 to the general problem of non-sexual reproduction 1 



