120 M. A. Giard on the Embryogeny 



to a much smaller cell, with finely granular protoplasm. Thus 

 is effected the separation of the plastic vitellus from the nutri- 

 tive vitellus. The plastic spherules have a nucleus and a 

 nucleolus, and they soon multiply rapidly j while the number 

 of large nutritive spheres augments, on the contrary, with ex- 

 treme slowness. The plastic sjoherules not only form a mass 

 at one point of the egg, as has been already described and 

 figured in Vermetns, \ but they invade and cover up all the 

 nutritive vitellus, forming the ectoderm. The nutritive spheres, 

 the division of which takes place less rapidly, give origin to 

 the endoderm. All this process resembles very closely that 

 which has been observed in certain worms (for example in 

 Euaxes) by Kowalevsky. 



After segmentation the first modification that takes place 

 is a thickening of the ectoderm, at a point near that where 

 this lamella finally closes (prostoma). This thickening be- 

 comes covered with vibratile cilia and hollowed by a cavity 

 (cephalic vesicle). At the same time the definitive mouth is 

 formed by an invagination of the ectoderm situated on the 

 anterior third of the embryo, below the cephalic vesicle. The 

 cephalic expansion soon divides into three lobes, a median and 

 two lateral lobes, forming a sort of trefoil open below where 

 the buccal opening is situated. The median lobe is covered 

 with very fine vibratile cilia ; the lateral lobes are bordered by 

 a row of large cylindrical cells provided with much longer 

 cilia. The embryo turns rapidly on itself in the mucus which 

 fills the nest. It absorbs the rudimentary eggs, and, even under 

 the microscope, the matters proceeding from the difBuence of 

 the neighbouring embryos, borne cells detach themselves from 

 the ectodermic lamella in the median lobe, and emit pro- 

 cesses which unite them on the one hand to this lamella and 

 on the other to the oesophageal invagination. This is the first 

 rudiment of the middle lamella that will produce the vascular 

 system. 



The lateral lobes soon become considerably developed, and 

 unite to form an irregularly quadrangular ciliated collar, of 

 which the lateral parts become forked and are formed at a 

 later period into elegantly pigmented vela. No traces of 

 tentacles are to be seen. 



The foot is derived from a thickening of the ectoderm situated 

 under the mouth ; this thickening is ciliated at its free extre- 

 mity. The nervous system appears under the form of an infla- 

 tion of the ectoderm situated on each side at the point of 

 junction of the lateral lobes with the cephalic vesicle ; the 

 two inflations afterwards approach the median line and are 



