248 EMBRYOLOGY. 



are, however, certain details of the process that are difficult to comprehend. 

 A brief summary ^ will make this fact clear. 



"The layer forming the primary invagination (? archenteron) does not 

 correspond to the definitive entoderm, but gives rise to all the tissues of the 

 trunk. By the tangential division of its cells, it gives rise to a superior ento 

 dermic mass resting upon a single layer of cells; the latter increases by the radial 

 division of its cells and becomes infolded, forming three invaginations ; of these 

 the middle one, which remains open, becomes the future proctodaeum, while 

 the two lateral ones close and are transformed into the masses of mesoderm, the 

 lateral mesoderm-bands. The lower layer, which now has the form of a vault, 

 represents the ectoderm of the trunk. The lips of the proctodaeum now grow 

 out to form the caudal button which first projects into the cavity of the ecto- 

 dermal vault, but later, together with the entire vault, becomes, evaginated 

 through the depression at the posterior end of the larva. This conical protu- 

 berance with the caudal button and the proctodaeum at its extremity represents 

 the trunk of the young Proneomenia. The entoderm still remains as a solid 

 mass with the mesoderm-bands on either side and in contact with the procto- 

 daeum behind. The next important change is the appearance of three ventral 

 invaginations of the larval ectoderm, just behind the circle of large cilia on 

 the middle segment; the median of these invaginations, the larval stomodaeum, 

 is merely transitory, while the two lateral ones are concerned in the formation 

 of the ectoderm and mesoderm of the head. These two unite, forming a trans- 

 verse band capping the anterior end of the entodermic mass and prolonged 

 posteriorly at two points to meet the mesoderm-bands of the trunk; this por- 

 tion appears to form the muscles, while the more dorsal elements of the invagi- 

 nation form the cerebral gangUa. The cells of the apical plate seem to take no 

 part in the formation of the nervous system. The ectoderm of the head appears 

 to form entirely from these anterior invaginations, while that of the trunk de- 

 velops from the primary posterior invagination. The latter is now completely 

 evaginated, and has developed the provisional imbricated spicules. In this way 

 the young Proneomenia is developed under cover of a provisional ectoderm 

 which serves as a locomotory organ and is thrown off at the moment of meta- 

 morphosis. The adult does not exhibit a distinct head but, during development, 

 this structure is perfectly distinct and arises quite independently of the trunk." 



Upon first thought it appears difficult to correlate some of these observa- 



' From a note by M. F. Woodward in the English translation of Korshelt and Heider's Text-book 

 of the embryology of invertebrates. 1900, 4, p. 19. 



