54 SUMMABY OF CUERENT RESEARCHES RELATING TO 



also observable, which tends to transport the spheres derived from the 

 small segment from the animal pole to the opposite one, skirting the 

 dorsal face, while the large spheres give place to them and glide along 

 the ventral face. 



At the stage xvi. the egg is composed of a row of four small cells 

 derived from the small segment and occupying the dorsal face, of four 

 spheres, larger and larger, occupying the ventral face, and of two 

 rows of four cells placed on the sides, and four derived from the large 

 and four from the small segment. 



It is only after this stage xvi. is reached that the dorsal and 

 lateral cells commence to multiply much more rapidly than the 

 ventral ones, and to spread over their sides. In proportion as these 

 small cells glide over the surface of the large ones, the latter sink 

 with an oscillatory movement, which at first removes the smaller 

 ones, until at length the last and largest glides in its turn under the 

 first, leaving an orifice, the blastopore, which remains visible for some 

 time almost exactly at the spot where, later on, the mouth is formed. 



By the very place which it occupies from the moment of the 

 closing of the blastopore, it is easy to see that the last sphere 

 enveloped corresj)onds to the intestine, which it will serve to form, if 

 not entirely, at least in great part. 



In the same way, by the manner of their inclusion, the two large 

 spheres following will be on the ventral face of the first, in the situa- 

 tion which the genital organs will occupy. Later on, when the 

 spheres begin to divide and subdivide, this disposition becomes very 

 obscure ; but for a certain time after the closing of the blastopore it 

 remains perceptible, and shows that the embryo is formed, if not of 

 continuous layers, at least of masses of tissue which obviously corre- 

 spond to the endoderm, mesoderm, and ectoderm of the higher animals 

 both in their position and destination. 



When the subdivision has been pushed to its furthest limit the 

 egg presents the form of a finely moruloid mass, in which can only 

 be recognized an outer light layer and a darker central one. The 

 cephalic region always remains lighter. The blastopore is no longer 

 distinguishable. 



Soon, along the side and ventral face an oblique furrow appears 

 which constricts the mass and separates the tail ; the latter is thus 

 folded under the ventral surface and directed towards the head, as in 

 the embryo of Brachionus and Pedalion. 



About the level of the caudal extremity a depression aj^pears in 

 the cephalic mass ; it is uncertain if it corresponds to that described 

 by Salensky in Brachionus, but it indicates the appearance not of the 

 mouth but of the vibratile pit situated under the liji in the adult. 

 A little later, and somewhat higher up, the mouth appears, as a 

 depression sufficiently sunk, without doubt to form the mouth, but 

 certainly not sufficiently to form the mentum. Yet later, and also on 

 the back, the cloaca is formed by an invagination of the ectoderm, 

 and this, though very long in the adult, is as yet very short in the 

 larva, and remains reduced to a simple cmargination in the Floscu- 

 larise. The cephalic region is soon defined by a slight fold, which 



