DEVELOPMENT OF THE PECTINI BRANCHI ATA. 345 



he rudiments of the two rounded lobes (velum) with the foot 

 k figs. 12 a, b, c, 13 & 14 b, c). Cilia quickly made their appear- 

 ance both on the foot and on the lobes : subsequently, scattered 

 cirrhi could be observed upon the lobes, and then the embryo 

 began to rotate. Later still the lobes and the foot increased in 

 size (figs. 15, 16 b, c), and the rudiments of the auditory organs 

 (fig. 16 d) appeared at their base, the membranes of the mantle 

 became more and more thickened, the shell began to be formed 

 on its most sloping part, and calcareous particles to be deposited 

 in it (figs. 14,15,16 a). The embryos whose first development we 

 have just been following were true monsters, and subsequently 

 assumed such various and whimsical forms, that no one could 

 have imagined them to be individuals of the same species. In a 

 few we have seen the salivary gland appear (fig. 16 e\ but it was 

 the only new structure which appeared after the formation of 

 the external organs, and these beings permanently remained in 

 the same state of arrest of development. Finally, until eight 

 weeks had elapsed, this monstrous embryo was always to be met 

 with in the capsule. We have already stated that an ovum of 

 this nature always existed in each capsule, and its embryo was 

 known at once by its small size and the excessive vivacity of its 

 motions. We have sought for them in vain in the capsule after 

 eight weeks, and we suppose that they had all perished. When 

 our attention was first directed to these simple ova, which had 

 regularly undergone the cleavage process, we imagined that their 

 development had taken place in a normal manner; but, far from 

 this, it was, in fact, an abortion. For the viability of the indivi- 

 dual organized, more than one ovum is necessary : and despite 

 the regularity and the vivacity observable in the young product 

 of the single ovum, we see that its development remains in the 

 highest degree incomplete. This single ovum had in fact under- 

 gone all the stages of cleavage, and to all appearance united all 

 the anatomical and physiological conditions necessary to its 

 complete development, while, on the other hand, it appears to 

 us to be incontestable, that it had never been in possession of the 

 materials requisite for the formation of organs. Without doubt 

 there is much that is obscure in these ideas ; we shall endeavour 

 by and by to throw as much light as we can upon them. 



Having described the monstrous embryo resulting from a 



