64 IltsceUaneous, 



period of the segmentation the behaviour of the nuclei and asters 

 is very remarkable. We also observe a movement of rotation 

 (already recognized by Barrois in Pedalion), vrhich tends to trans- 

 port the spheres derived from the small segment from the animal 

 pole to the opposite pole, skirting the dorsal surface, while the large 

 spheres give place to them and glide along the ventral surface. 



At the stage XVI. the egg consists of a row of four small cells 

 derived from the small segment and occupying the dorsal surface, of 

 a row of four spheres gradually increasing in size occupying the 

 ventral surface, and of two rows of four cells placed on the sides, 

 four of them derived from the large and four from the small seg- 

 ment. 



It is only after this stage XVI. that the dorsal and lateral cells 

 begin to multi})ly much more rai)idly 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 latler sink by an oscillatory move- 

 ment, which at first removes the smaller ones, until finally the last 

 and hirgest of them slips in its turn beneath the former ones, leaving 

 an orifice (the blastopore), which remains visible for some time, 

 almost exactly at the spot where the mouth will afterwards be 

 formed. 



Even by the situation it occupies from the moment of the closure 

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

 corresponds to the intestine, which it wiU serve to form, if not en- 

 tirely, at least in great part. 



In the same way, by the manner of their inclusion, the two fol- 

 lowing large spheres will be upon the ventral surface of the former, 

 in the situation that will be occupied by the genital glands. Sub- 

 sequently, when the spheres come to divide and subdivide, this 

 arrangement Avill become very obscure ; but for a certain time after 

 the closure of the blastopore it remains perceptible, and shows that 

 the embryo is formed, if not of continuous lamellie, at least of 

 masses of tissue which obviously correspond to the cndodcrm, meso- 

 derm, and ectoderm of the higher animals, both by their position 

 and their destiny. 



When the subdivision has been carried to its last limit, the e^g 

 appears as a finely moruloid mass, in which we can recognize 

 only an outer light layer and a dark central one. The cephalic 

 region always remains lighter. "We can no longer distinguish the 

 blastopore. 



Soon afterwards an oblique furrow, which constricts the mass and 

 separates the tail, appears on the side and along the ventral surface ; 

 the tail is thus folded under the ventral surface and directed towards 

 the head, as in the embryo of Brachioaus and Pedalion. 



About the level of the extremity of the tail a depression appears 

 in the cephalic mass. I do not know whether this corresponds to 

 the depression described by Siiiensky in Bra chi onus ; but it indicates 

 the appearance, not of the mouth, but of tlie vibratile pit situated 

 below the lip in the adult. A little later, and a little higher up, the 

 mouth makes its appearance as a depression wliich no doubt sinks 



