No. 2.] THE EMBRYOLOGY OF LIMULUS. 21 7 



which are to the effect that the yolk cells are " vitellophags," 

 degenerating and not contributing to the formation of the 

 entodermal epithelium which arises from cells derived from 

 some other source. I feel confident that this is not the case 

 in Limulus. I have yet to see any evidence of degeneration 

 of the yolk cells, and further, I have seen no cells other than 

 those of the yolk which could supply the epithelium of intestine 

 and liver. From the very time when the yolk is first included 

 in a mesodermal envelope this layer is to be clearly traced (jt/.) 

 as a splanchnoplure closely enveloping the yolk, and in the 

 later stages the same layer is seen (Figs. 84, 85) in exactly the 

 same place and presenting the same conditions. In some 

 sections which I have made of Hexapod eggs I have seen 

 appearances which lead me to think that possibly in these 

 forms the entoderm of authors is in reality splanchnoplure. 

 The fact that most observers have closed their investigations 

 before the development of a well-differentiated entodermal 

 epithelium leaves a gap which renders their interpretations not 

 conclusive on this point. 



Stomod^um. — As already described (Vol. VII. p. 52) the 

 anterior end of the primitive streak is marked by a spot where 

 the cells are deeper and more columnar (Figs. 43, 45, vio.) 

 than elsewhere in the median line, and this spot is usually, in 

 Arthropods, regarded as marking the position of the future 

 mouth, and all discussions of the transfer of the mouth from 

 a preappendicular position to one behind the first pair of ap- 

 pendages are concerned with the connection of this spot with 

 the mouth of the adult. In reality this spot marks the junc- 

 tion of stomodaeum and mesenteron and forms the inner end 

 of the foregut. Hence to call it the mouth is to introduce 

 an element of confusion. 



At the time of the first outlining of the limbs the invagina- 

 tion of the stomodaeum begins, and the process is one which 

 finds its closest parallel in the invagination of the neural canal 

 in the vertebrates.^ At first it is a shallow pit with small 



1 It is needless to say that this affords no foundation for the curious vagaries 

 of Gaskell, as to the origin of the vertebrate nervous system from the alimentary 

 tract of the Arthropod. 



