ON THE DEVELOPMENT OF SAGITTA. 363 



derm give rise to the longitudinal muscles of the body, and 

 that while the greater part of the cell-substance is trans- 

 formed into muscle, the nucleus and a little protoplasm 

 remain at the inner end and ultimately form tlie lining of the 

 ccelom. This will be further described in dealing with the 

 sections of larvte. 



It will be convenient at this point to describe more fully 

 the four genital cells, which have now reached their per- 

 manent condition. They lie one behind the other in the 

 ccelomic cavit}- on each side, and when this is obliterated 

 they become embedded in the mesoderm and lie at the sides 

 of the endodermic septum. The male and female cells appear 

 exactly alike, and are characterised by their extremely lai-ge 

 nuclei, each of which is generally oval in shape, is enclosed 

 by a definite membrane, and contains numerous nucleoli. 

 The latter are arranged round the edge of the nucleus, close 

 to the nuclear membrane, and they are generally of a rather 

 elongated oval shape. In the remainder of the nucleus a 

 network of fine threads can generally be made out, and since 

 only the nucleoli take up the stain, in some sections the 

 genital nuclei appear to be made up of a large number of 

 very small cells, each with a stained nucleus. Closer exami- 

 nation with a high power, however, shows that this is not the 

 case ; the genital nuclei are exactly like those of the rest of 

 the body, except that they are more than twice as largo, and 

 the nucleoli are less crowded together. The cell protoplasm 

 of the genital cells is small in amount, and its limits are hard 

 to see, for when embedded in the mesoderm they become 

 enclosed in a sort of envelope of mesodermal cells, which, 

 however, do not at this stage form a definite epithelial 

 sheath. In some sections this mesodermal envelope is not 

 conspicuous, but in others (PI. 19, fig. 15) small nuclei can be 

 seen closely appressed to the genital cells, and these differ 

 from the other nuclei of the body in staining evenly throuo-h- 

 out, instead of consisting of a mass of nucleoli embedded in a 

 colourless matrix. 



The ectoderm of the trunk is chq,racterised chiefly by the 



