CRANGON VULGARIS. 27 



segments. In sta^e Q the abdomen is short and the anus 



O O 



is still dorsal in position. A little later, staged, it is termi- 

 nal and plainly (fig. 51) these budding cells are adding to 

 the length and taking part in the transposition of the anus 

 from the dorsal to the ventral surface of the body. In this 

 condition it is to be noted that these germ cells belong to 

 both ecto-and mesoderm and thus present close resem- 

 blances to the "neuroblasts" and "mesoblasts" of Dr. Whit- 

 man. The number of these cells vary. In transverse section 

 I have seen as many as eight in either layer at the tip of the 

 abdomen, but not so many in the thoracic zone of growth. 



It is not until stage F that there begins to be much dif- 

 ferentiation of the mesoderm, but it is to be noted that at 

 this, as well as later stages, the mesoderm of the append- 

 ages is solid and I have not been able at any time to rec- 

 ognize a well defined crelom. Occasionally cavities, formed 

 by splitting, in the middle layer but whether these are to 

 be recognized as schizoccelia or as blood vascular spaces 

 cannot be decided by any evidence at hand except on the 

 theoretical grounds advanced by Lankester ('88 and elab- 

 orated for Peripatus by Sedgwick ( J 88). 



THE HEART. The heart begins its beating in stage F; 

 it is then but a linear tube which becomes obliterated in the 

 sections,' and I have no information as to whether it arises 

 by a splitting of the mesoderm or as a remnant of a theo- 

 retical segmentation cavity as suggested by Biitschli and 

 Schimkewitsch. My sections unfortunately show almost 

 nothing of its development except that it appears as a dif- 

 ferentiation of a slender dorsal vessel arising in the dorsal 

 mesoderm and apparently (fig. 54) in that portion usually 

 recognized as splanchnoplure. In stage If the heart has be- 

 come well developed and, as observations on the living em- 

 bryo show (fig. 21), consists of but a single layer of cells 

 with a single ostium on either side. In the early stages 

 the ophthalmic artery is the most conspicuous, appearing as 



