EMBRYOLOGY OF CLEPSINE. 275 



spparation. As soon as the posterior sucker is developed they 

 attach themselves by this to the mother, the body and head 

 swinging free. 



At the time of exclusion the neuroblasts have diminished 

 much in size. They continue the process of proliferation for at 

 least a day or two after hatching. Remains of the neuroblasts are 

 seen three days after they leave the egg-envelope (fig. 50) , but they 

 have ceased to make contributions to the embryo, and are soon 

 lost in the yolk. The mesoblasts continue their activity for 

 about the same period, and finally blend with the yolk. 



h. Pharyngeal Clefts. — The two depressions {oc) noted in 

 stage 41, which, starting from the primitive groove, pass right 

 and left between the fore ends of the germ-bands and the cephalic 

 portion, are in stage 46 extended around a circular area destined 

 to become the protrusible pharynx. Stages 48 and 50 show that 

 these depressions are the incipient invagination of the pharyngeal 

 atrium (o a). 



These clefts are very distinct on specimens hardened in 

 chromic acid, and, after they have been once seen on such pre- 

 parations, are recognised without difficulty in a fresh condition. 

 A more advanced stage of the invagination is seen in fig. 96, o a. 



The permanent mouth is the pharyngeal orifice (jt? a). Both 

 atrium and mouth are ectodermal invaginations. 



G. Nerve-chain. — Although my investigations here suffice only 

 to form a basis for more detailed study, they settle a point of 

 cardinal interest, namely, the precise origin of the neural ele- 

 ments. In addition to what has been said on the origin and 

 composition of the germ-bands, it remains only to consider sec- 

 tions of later stages. 



Fig. 84 represents a median transverse section of stage 41. 

 The origin of the large mesoderm cells is here placed beyond all 

 doubt. The same is equally clear in section 85, which shows 

 some of the anterior neuroblasts. On each side is seen a line of 

 small cells leading from a neuroblast. In fig. 84 the superficial 

 part of each germ-band consists of four of these neuroblastic 

 productions. It is these cells that give the germ-bands their 

 fourfold striated appearance. The same cells are seen again in 

 fig. 87, which is a horizontal section of an embryo in stage 47. 

 The section passes just under the neuroblasts and cuts the two 

 unclosed ends of the bands. In the anterior portion of the 

 section, where the germ-bands have united, the nerve-cells form 

 a line of eight cells lying just under the epidermis. The expan- 

 sion of the ectoderm is more rapid than that of the bands, and 

 hence the epidermis comes to cover the nerve-cells, and even 



