THE DEVELOPMENT OF ECHIXORHYNCHUS. 557 



ill each. The two series meet near the middle line to form 

 an arch, the central and largest spine constituting its summit. 

 Two short, ridge-like elevations of the cuticle, close to the 

 middle line, separate the spines on either side from one an- 

 other. Behind, the peripheral layer gives rise to a knob-like 

 process. 



At tlie end of fourteen days, the embryo is found to have 

 increased much in size, but jDresents few changes of form. 

 The anterior extremity displays two rounded elevations, the 

 spines retaining their original position. The peripheral layer 

 has become thicker and more distir.ct ; its knob-like process 

 has by this time disappeared. The central mass, now much 

 larger, has assumed a spherical figure. No longer granular, 

 it is seen to be composed of numerous pale cells, which con- 

 tinue rapidly to increase. 



During the third week, numbers of large yellow granules 

 begin to appear within the outer layer of the embryo. No 

 other changes, save those of growth,' take place in its walls : 

 but the central mass, still continuing to enlarge, gradually 

 puts on the aspect of a young Ecliinorhyncliys. This mode 

 of development has been compared by Leuckart to that of 

 certain Echinoderms, or to the production of tlie Nemertid 

 larva within its pilidium. 



The first part to become differentiated is the cavity of the 

 future proboscis, which appears as a transparent lenticular 

 vesicle at the anterior end of the spherical mass. Behind this 

 are soon seen rudiments of the central axis and its contained 

 ganglion ; and the suspensorial ligament, with the reproduc- 

 tive organs, are, at the same time, marked out. The muscles 

 of the outer wall have also commenced their development. 

 Next, the central region of the young Echinorhynchus rapid- 

 ly elongates ; its walls become thinner, and, separating from 

 the included structures, show the first trace of the visceral 

 cavity. About this time distinctions of sex first make them- 

 selves evident. The posterior end of the body undergoes a 

 disproportionate increase of size, the muscles become more 

 distinct, and the rudimentary generative organs are clearly 

 manifest. At length the young Echinorhynchus occupies 

 almost the whole interior of the embryo, the walls of which 

 have, meanwhile, undergone but slight histological change. 

 The spines, however, have disappeared, together, it would 

 seem, with the cuticle to which they were attached. No rup- 

 ture of the other embryonic structures takes place, but they 

 gradually attach themselves to the body of the contained 



