ON THR DKVRLOPMENT OF SAGITTA. 375 



this age. The most prominent objects in the first two of 

 these are the vestibular ganglia lying at the sides of the 

 month, which are now almost completely developed, and 

 consist of an inner mass of '' Punktsubstanz/' surrounded by 

 a layer of deeply staining nuclei. 



The cerebral ganglion is also complete by this time ; it lies 

 at the anterior extremity of the head and to a large extent in 

 the hood, in which it extends ventrally nearly as far as the 

 mouth. In the adult it becomes more restricted, as does the 

 abdominal ganglion. At this stage it consists of an outer 

 layer of nuclei covering a deeper mass of ''Puuktsubstanz," 

 but the latter is more restricted than the nuclear layer. Fig. 

 25 shows on the dorsal surface of the mouth region two large 

 masses of nuclei in the ectoderm, representing the eyes (e.), 

 while in this figure and in fig. 24 the hooks are seen lying at 

 the sides of the head, covered by the hood {hks.). 



The development of the mesodermal structures has now 

 proceeded considerably, and the chief muscles of the head 

 are already differentiated. Their general arrangement is 

 indicated in fig. 26 {h. mus.), where it is seen that they do 

 not differ much from those of the adult. Behind the mouth 

 the pharynx lies near the dorsal surface, and when the 

 muscles are formed a cavity appears just below it, which 

 corresponds with the cavity called by Hertwig the head 

 ccelom. A little fui'ther forward a cavity is present on each 

 side more dorsally (figs. 24, 25, b. c. 1.), and these are 

 undoubtedly coelomic. Already about the third day the 

 pharynx is seen to be surrounded by a layer of cells which 

 belong to the mesoderm, corresponding to the splanchnic 

 layer (fig. 23), but, as in the trunk, these cells later be- 

 come so closely connected with the alimentary canal as to 

 be indistinguishable from it. About the fifth day, however, 

 a cavity appears on each side between these cells and the 

 outer layer of cephalic mesoderm, which is now being 

 changed into muscles. This cavity is the head coelom, and 

 that below the pharynx further back appears to be also 

 coelomic, and to be formed by the coalescence in this region 



