378 E. RAY LANKESTER. 



question as to whether the invaginate gastrula of Lymnceus 

 forms by emboly or epiboly, or has an intermediate 

 character. 



The Trochosphere. — The orifice of invagination of the 

 gastrula now closes up, and its shape commences to undergo 

 a change due to the development of a kind of equatorial 

 ridge, the earliest rudiment of the velum. At the same timi^ 

 the movements of rotation of the embryo commence. The 

 phase in which there is as yet no trace of the mouth, and 

 in which the gastrula's orifice of invagination has disap- 

 peared, is not figured in the plates accompanying this paper ; 

 but I may refer to Lereboullet's pi. xii, fig. 36, for a good 

 drawing of that particular phase, though the French natura- 

 list does not recognise the significance of his illustration, 

 since he believes that the Gastrula's orifice becomes the 

 mouth. 



The movements of rotation in the embryo LymncBus are 

 caused by very short cilia, which it is not difficult to see 

 even with a quarter-inch (English make), after the embryo 

 has been treated with osmic acid. These cilia have entirely 

 escaped M. Lereboullet, who says, " J'ai cherche en vain la 

 cause de ce mouvement qu'on attribue g^neralement a des 

 cils vibratiles. Je puis affirmer que ces derniers n'existent 

 pas, et qu'ils ne se voient jamais, a aucune epoque de la vie 

 embryonnaire, sur toute la surface de I'oeuf." 



With a No. 10 a immersion Hartnack the cilia can be 

 observed, even in the early period, when rotation first 

 begins ; later they are obvious enough in the region of the 

 velum. 



The phase which the embryo now enters upon with a 

 distinct circumferential ciliated band is that which I have 

 designated in the introductory remarks above as the trocho- 

 sphere. In the earliest of the forms referable to this phase 

 (PI. XVI, fig. 13) the embryo has a very peculiar outline when 

 viewed from the oral pole, the ciliated band appearing to 

 commence its development in connection with the two lobe- 

 like outgrowths right and left of the mouth. The remaining 

 figures on PI. XVI give various views of later trochospheres. 

 The movements of rotation are now very rapid, and vary 

 around two axes at right angles to one another, so that it is 

 difficult to get a correct notion of the actual superficial form 

 of the embryo. The figures supply such information as I can 

 give. 



The changes in the histological elements of the embryo 

 from the earliest gastrula-form (fig. 7) to the latest trocho- 

 sphere are no less marked and important than the changes in 



