290 E. W. MACBRIDE. 



of the ovoid gland. Figs. 102 and 103 show that they do not yet 

 extend through its whole extent. Fig. 103 shows that the ovoid gland 

 rudiment is at one point attached to the oral wall of the axial sinus. 

 (Compare Plate 25, fig. 110.) 

 Figs. 104 — 106. Three sections of a young star-fish, in which U equals 

 ■4 millimetre. Fig. 104 shows the primitive germ cells arising from 

 the involution of the peritoneum. Figs. 105 and 106 show that they now 

 extend throughout the whole extent of the ovoid gland ; these figures 

 also show the relation of the oral end of the axial sinus to the peri- 

 haemal spaces. 

 Figs. 107 and 108. — Two sections from same series as Figs. 82 — 84, 

 magnified 350 diameters. They show the development of the oral "blood" 

 ring, sang, circ, as a modification of the mesenchymatous tissue of the 

 blastocoele. ^br. Fibrous tissue. 



Fig. 109. — Similar section of a young star-fish, in which R equals "45 

 millimetre. Same magnification. The blood-ring is fully formed. Notice 

 also the minute cells amongst the nerve-fibres (bi^. gang). 



PLATE XVIII. 



Fig. 110. — Longitudinal section of the stone-canal of young star-fish, in 

 which R equals "8 millimetre, sang. circ. Oral "blood" ring. wvr. Water- 

 vascular ring-canal, muse. amb. Muscles of ambulacral ossicles. Notice the 

 incipient division of madreporic pore into two, and entire independence of 

 ovoid gland and blood-ring. Magnified 350 diameters. 



Fig. 111. — Diagram showing the relative positions of the ovoid gland, 

 stone-canal, and various sinuses in proximity, gen. r. Genital rachis. ab. 

 Aboral sinus (or sinus a.), pr. germ. inv. Primary peritoneal involution to 

 form germ cells. The cavity of this is probably the same as sinus b in next 

 figure. The axial sinus a' is sinus e. The dotted lines show the continuity of 

 two parts of the ovoid gland in a different plane to that of the stone-canal. 



Fig. 112. — Similar diagram of Amphiura squamata. Accompanying 

 spaces, sinus a, sinus b, and sinus c, as in the author's paper (14). 



Figs. 113 — 118 illustrate the development of the ovoid gland and genital 

 rachis. They are all taken from sections cut parallel to the disc ; they are, 

 in fact, transverse sections of the interradial septum in which the axial sinus 

 is embedded. 



Figs. 113 and 114. Two sections from a star-fish, in which R equals "45 

 millimetre. Fig. 113 shows the manner in which the right hydro- 

 cffile is enclosed in the upper part of ovoid gland ; Fig. 114, the 

 primitive peritoneal involution, the pore-canal, and the crescentic form 

 of right hydrocoele. Magnification 350 diameters. 

 Figs. 115 and 116. Two sections from star-fish, in which R equals 

 7 millimetre, ab. Aboral sinus containing the rudiment of genital 



