242 GILBKRT C. BOURNE. 



recesses are, of course, lined by ectodermal epitlieliiim, and 

 at their inner ends the epithelium is prolonged into a number 

 of ramified ingrowths, which penetrate deeply into the 

 plerome in ranch the same manner as the allantoic vilH 

 penetrate into the trophoblast of a mammal. Fig. 25 is an 

 enlarged drawing of the area indicated by the reference 

 number 25 in fig. G. It is not sufficiently magnified to show 

 details, but it exhibits the general relations of the tissues of 

 this region, showing the powerful muscular bands of the 

 sphincter, the longitudinal muscle-bands, /. 71111.'^., inside of 

 these bordering on the cavity .'", and numerous well-defined 

 blood-spaces, hi. The intrusive ectodormic ingrowths, ec. L, 

 are distinguished by their numerous dark nuclei, and the 

 drawing gives a good idea of their number and extent, and of 

 the relations they bear to the other tissues. Fig. 30 is a 

 highly magnified drawing of a portion of fig. 25, showing the 

 ectodermal ingrowths, ec. i., formed of the characteristic 

 pigmented ectoderm-cells, many of which contain yellow 

 gi-anules. The ingrowths penetrate into the blood-spaces, 

 and there can be little doubt that the whole structure is 

 subservient to the nutrition of the test. 



The two tubes marked vp. in the same figure, whose walls 

 are composed of one or more layers of cells with deeply 

 stained nuclei, are cross-sections of two of the tassel-like 

 vascular processes described above as penetrating the sub- 

 stance of the test in all parts of the body. 



The general appearance of these vascular processes is best 

 shown in fig. 5, vp. They are most abundant on the arms, but 

 occur over the whole surface of the body, and, as the body- 

 wall has shrunk away from the test, souie of them have been 

 pulled oat of the crypts into which they fitted and remain 

 adherent to the body-wall ; others have been torn away, and 

 remain embedded in the test. They vary very much in 

 length ; those on the arms are quite short, as shown in fig. 5, 

 but in other parts of the body they may be of considerable 

 length, penetrating for a long way into the substance of the 

 test. Each is a simple tubular outgrowth containing a cavity. 



