DEVELOPMENT OF PHETGANIDS. 673 



The second furrow gives rise to a part of the nervous system^ and 

 hereafter we shall call it the "neural furrow/' 



The gastrula then is quite open at first; but its lips gradually 

 close, so that the only trace of the opening left is that shown 

 by the arrangement of the invaginated cells and their nuclei 

 (PI. XXXVlB,fig. 10). The closing of the gastrula mouth begins 

 at the posterior end of the body, and gradually extends towards 

 the head end. In no case do the infolded cells form a closed 

 tube. In two or three hours the furrow has entirely disap- 

 peared. 



The cells which are thus permanently cut off from a direct 

 share in forming the outer wall of the body constitute the be- 

 ginning of the mesoderm. Although the boundaries of these 

 cells are no longer sharply defined, the position of their nuclei 

 shows clearly the relation which they sustain to the rest of the 

 ventral plate, and is suggestive of the stages which have im- 

 mediately preceded. As seen in Plate XXXVIb, fig. 10, these 

 nuclei are already arranged in two rows approximately parallel 

 with the nuclei of the neighbouring ectoderm. "With the de- 

 pression of the dome-shaped ventral plate which occurs at this 

 period, the mesoderm is changed into a single layer of cells 

 covering the under surface of the ventral plate (PI. XXXVIb, 

 fig. 11). This movement, however, may not necessarily imply 

 any marked amoeboid properties for the cells at this period ; they 

 have increased in numbers, and the shifting incident upon cell 

 division has played an important part in the changes of position 

 that have been observed. 



Zaddach's description of the splitting of the ventral plate into 

 a *' Hautblatt " and " Muskelblatt " can in no way be com- 

 pared to the formation of the mesoderm as described above. 



at least a part of the endoderm does arise from invaginated cells. But even 

 if this was not so, I cannot see that it would influence our use of the term iu 

 this case. For it seems to me that an inherited tendency to produce an 

 invagination, the sole object of which is to cause a differentiation into germ 

 layers, however successful or unsuccessful the attempt may be, should receive 

 the name that was originally applied to the more simple condition, always 

 holding in mind, however, the fact that it is a condition modified by various 

 agents, the nature of which may or may not be known. 



