250 EMBRYOLOGY. 



on all sides by the test. The same effect may be produced in Halomenia (Plate 

 13, fig. 11 for example) by slightly extending the borders of the test. The 

 layer forming this "primary invagination" is therefore no archenteron, and 

 Pruvot is quite correct in claiming that its cells do not correspond to the defini- 

 tive endoderm and that they give rise to all the tissues of the trunk. By tan- 

 gential divisions of the cells of this depressed area there is produced a "superior 

 ectodermic mass resting upon a single layer of cells," that is to say the ectoderm 

 of the trunk becomes distinct from the endoderm that later forms the mid-gut 

 (Plate 14, fig. 5). 



In this area circumscribed by the borders of the test are "three invagina- 

 tions; of these the middle one, which remains open, becomes the proctodaeum, 

 while the two lateral ones close and are transformed into "masses of mesoderm." 

 The proctodaeum is evidently the mid-gut, but that it is open to the exterior 

 or is derived from this species of invagination is certainly an erroneous con- 

 clusion resulting from a failure to detect the true blastopore. The mesoderm 

 bands are evidently the ventral cords of ganglion cells as will appear more 

 clearly later on. Concerning the formation of the trunk it may be said that 

 the "Ups of the proctodaeum" evidently refers to the rim of the terminal depres- 

 sion in the trunk of comparatively old larvae and the "terininal button which 

 first projects into the cavity of the ectodermal vault" probably refers to the 

 group of cells that in one stage in the development of Halomenia (Plate 14, 

 fig. 4) lie at the base of this depression. Later the button "together with 

 the entire vault becomes evaginated" beyond the borders of the test, and the 

 presumption is that the button corresponds therefore to the dorso-terminal 

 sense organ. 



In the anterior half of the body three invaginations are said to exist in 

 the midst of the test cells. The first, a transitory structure, is said to repre- 

 sent the stomodaeum. If such is actually the case it occupies a very different 

 position from what it does in Halomenia. That it is transitory makes it appear 

 to be a misinterpretation. The two lateral invaginations that ultimately meet 

 to form a transverse band are said to supply material for the ectoderm of the 

 head, the cerebral gangUa and the mesoblastic bands that are "prolonged pos- 

 teriorly to meet the mesoderm-bands of the trunk." I am not certain regarding 

 the head ectoderm formation but the mesoderm bands are clearly the ganglionic 

 trunks that continue to the posterior end of the body. 



