CRANGON VULGARIS. 27 



Segments. In stage ö the abdomen is short and the anus 

 is still dorsal in position. A llttle later, stage E^ 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 "nenroblasts" and "mesoblasts" of Dr. Whit- 

 man. The n amber of these cells vary. In transverse section 

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

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



It is not uiitil stao^e i^that tliere beo^ins to be mach 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 coelom. Occasionally cavities, formed 

 by splittino^, 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 band except on the 

 theoretical grounds advanced byLankester (^88 and elab- 

 orated for Peripatus by Sedgwick (^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 Bütschli and 

 Schimkevvitsch. My sections unfortunately show almost 

 nothiiig 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 ZT the heart has be- 

 come well developedand, as observations on the livingem- 

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

 with a Single ostium on either side. lu the early stages 

 the Ophthalmie artery is the mostconspicuous, appearing as 



