Mr. C. Darwin on the Structure and Propagation of Sagitta. 5 



swellj and soon assumed the form shown by (C). Whilst this was 

 going on, the small internal globule also appeared to be swelling, 

 and at the same time the transparent fluid with which the ovum 

 and its enlarged apex were charged, became more and more opake 

 and granular. The apex continued enlarging until it became of 

 nearly the same size with the ovum from which it proceeded ; and 

 as this took place, all the granular matter was slowly expelled 

 from the original capsule into the newly-formed one, in a manner 

 which seemed to show that it was effected by the contraction of 

 a lining membrane as represented at (D) . Directly that this was 

 completed the two balls slowly separated ; one being left a mere 

 empty husk, and the other consisting of a spherical mass of gra- 

 nular matter, within which a minute globule could be discovered. 

 I presume that this was the same globule as seen within the egg 

 in its first state (as at B), and that the appearance of its swelling 

 was caused by the transparent fluid round it being first converted 

 into granular matter. I have reason to suppose from what fol- 

 lows that this little globule contains only air. The whole phse- 

 nomenon was effected in about ten minutes ; and in one case I 

 watched the entire process without taking my eye from the mi- 

 croscope. 



On the 27th and 29th of September 1832, we passed* through 

 the same tract of sea (off Bahia Blanca on the coast of northern 

 Patagonia) where twenty-five days previously I had observed such 

 great numbers of the >S^. exaptera with their ovaries distended 

 with eggs, and I now found infinitely numerous ova floating on 

 the surface. They were in different states of maturity ; those 

 least developed presented a sphere of granular matter contained 

 wdthin a larger spherical case. In the next stage the granular 

 matter collects in a linear manner on one side of the inner sphere, 

 and projects slightly beyond its outline ; it then soon forms a di- 

 stinct prominent rim, extending round two-thirds of the circum- 

 ference of the inner sphere. This prominent rim is the young 

 animal ; a fine vessel is seen extending within its entire length, 

 and one extremity enlarges into a head : the tail is first liberated 

 from its attachment on the surface of the inner sphere, and lastly 

 the head : the young animal, when thus released, lies in a curved 

 position within the outer case, with the inner sphere, on the cir- 

 cumference of which it was developed, pushed on one side, and its 

 function apparently ended. The central intestinal vessel is now 

 much more distinct : an excessively fine membrane-like fin is 

 discernible round the end of the tail; and the young animal 

 being Kberated from the outer spherical capsule, progresses by a 



* I may add, that in the beginning of April, off the Abrolhos, on the coast 

 of Brazil, inlat. 18° S., numerous specimens of a four-fiimed Sagitta had their 

 ovaries filled with eggs apparently ready to be expelled. 



