DEVELOPMENT OP AMPHTURA SQUAMATA. 137 



the stone canal. This is the first rudiment of the 

 ovoid gland. The coelom can be seen to run into a chink at 

 the distal end of the stone canal ; a single thread of plasma 

 stretches across it. This chink is the commencement of 

 the axial sinus (sinus h), which is thus a diverticulum 

 of the coelom. The ampulla (sinus c) is seen to be quite 

 distinct, and by comparing figs. 4 b and 4 c may be seen to 

 communicate with m. p., the madreporic pore. 



In fig. 5 a, and still more distinctly in fig. 5 b, we see an in- 

 crease in the number of nuclei forming the rudiment of the ovoid 

 gland. At the point where a mesenteric bridle goes off to the 

 stomach they form more than one layer, and show the charac- 

 teristic staining properties of the primitive germ-cells. The 

 axial sinus is clearly seen, and though its lumen is the merest 

 slit, its continuity with the coelom can easily be traced by its 

 walls. 



In the next stage (figs. 6 a, b, c) the multiplication of the 

 nuclei forming the ovoid gland is very marked, and they pro- 

 ject as a lobe which encloses under it a nook, so to speak, of 

 the ccelom. This is the first rudiment of the aboral 

 sinus, which, like the axial sinus, is an involution of the 

 ccelom, but quite a distinct one, though of course both at this 

 stage open freely into the coelom. 



The next and most important stage is represented in figs. 

 7 a, b, c. Here the ovoid gland has increased in size, and 

 has grown round the stone canal, so that in the upper part of 

 the section it appears both external (i. e. towards the mouth) 

 as well as internal to it. In the dorsal portion of the gland 

 the nuclei are spaced widely, and the characteristic plasma is 

 beginning to appear. In the ventral portion the nuclei are 

 very large, and are, in fact, becoming the primitive genital 

 cells, the " Urkeimzellen." Sinus b is now distinctly closed 

 off by a double membrane from sinus a, which opens freely 

 into the coelom. Figs. 7 b and 7 c show, however, that sinus 

 a is prolonged laterally as an involution, and ends in a solid 

 knob of cells. This is the beginning of the lateral outgrowth 

 of the aboral sinus. It is remarkable that it takes place only 



