lO- 



(a) Einbryos of C. Icvinseiii. 



The oldest embryos (PI. XI\', figs. 198, 199) usually have an ovoid form, their greatest 

 length being about 550 [x. The most striking facts which can be made out in the entire embryo 

 are : — (I) that the yolk is practically confined, at this stage, to a central mass, which is 

 distinctly divided into two regions ; (II) that a large area ot" the ectoderm of one surface of the 

 embryo is composed of cells which stain more readily than their neighbours; (III) that a part of 

 the ectoderm near this differentiated region contains numerous, highly refractive, rod-Hke bodies. 



Several frontal sections through an advanced embryo are shewn in PI. XIV, figs. 207 — 210. 

 It can hardly be doubted, from an inspection of these figures, that the region (vac^ containing 

 the refractive bodies is the anterior end of the embryo. The part indicated above by (II) appears 

 to be the ventral side, and I shall speak of it as the "ventral thickening" {v. t.). lts general 

 shape is shewn in fig. 207, which is a frontal section cutting the whole of this "ventral" region. 

 The subsequent sections (figs. 208, 209) shew that the ventral thickening extends round the 

 anterior end of the embrj-o towards the dorsal surface, but that it disappears before it reaches 

 that surface. 



Figs. 203 — 205, from a transverse series, shew more of the characters of the ventral 

 thickening, which contains a large proportion of much elongated cells of a glandular appearance. 

 This tissue closely resembles that which composes the anterior, thickened wall of the proboscis 

 of the adult. In both cases, the elongated gland-cells constitute the greater part of the epithehum, 

 and moreover have exactly similar staining properties. Thus the gland-cells, of the adult proboscis 

 and of the embryonic ventral thickening alike, stain intensely with Heidenhain's haematoxylin 

 or with "Orange G". It accordingly appears probable that the ventral thickening of the embryo 

 is in some way converted into the glandular epidermis of the proboscis of the adult. If this be 

 the case, it may perhaps be inferred that the embryo comes to rest, after a short free-swimming 

 period, and secretes the commencement of the future coenoecium by means of the gland-cells 

 in question. 



The view just indicated with regard to the further history of the ventral thickening 

 receives further support from the embryo represented in fig. 200. The part ivac?) containing the 

 refractive bodies is much elongated, a condition which appears to be due to the fact that the 

 embrj'o is squeezed into a narrow space between a zooid and the wall of its tube. The ventral 

 thickening is closely applied to this wall, in a way which suggests that the embryo was crawling 

 on it. The resemblance of these parts of the embryo to the proboscis of the adult is very 

 striking, and it is rendered more significant by the fact that the free dorsal and lateral edge 

 of the adult proboscis (PI. I, fig. 5) contains refractive bodies somewhat similar to those of the 

 embryo, although not behaving in quite the same way with varying modes of illumination. 



The yolk-containing central mass of these embryos has a highly characteristic form, which 

 may be described as umbrella-shaped (figs. 198 — 201). Distinct indications of a central cavity 

 may be found in the median part of this mass (figs 198, 199), which is sharply marked off 

 from the anterior part by the absence of yolk-globules in the intervening tissue. The whole of 

 the rest of the umbrella-shaped central mass is crammed with numerous yolk-globules, which 

 have a diameter of about 3 — 4 [j.. 



