Structure and Development of the Nephridia. 373 ■ 



distensiou of the body -wall by the contained coelomic fluid, whose 

 osmotic pressure must necessarily exceed that of the sea-water 

 (otherwise collapse would resultj. The extension of the body- wall 

 in the region immediately anterior to the growing zone is certainly not 

 due purely to a process of cell-multiplication or simple growth. This 

 is shown by the fact that the wall beeomes thinner and the nuclei 

 more sparsely distributed at the same time as the superficial exteut 

 increases, indicating clearly that the material which is being added 

 at the growing zone is undergoing a re-arrangement and extension 

 of a kind similar to that which would result from mechanical stret- 

 ching. The expansive force which in this manner Stretches the newly 

 formed body-wall and thus causes this region to extend both trans- 

 versely and longitudinally (see Plates 2i— 24, Figs. 9, 11, 29—32), is 

 in ali probability the osmotic pressure of the coelomic fluid. The 

 process may be compared roughly to the formation of a soap-bubble, 

 where the thick layer of fluid added at the rim of the pipe-bowl 

 (corresponding to the growing zone) is thinned-out and extended in 

 both directions by the pressure acting on the walls from within. 

 The analogy ueed not be pursued in further detail; it is probably 

 accurate to a certain degree, although in the growth of the larvai 

 body-wall material is undoubtedly added at other regious thau at 

 the growing zone. The analogy fails for example to explain why 

 the ventral body-wall remains thicker than the dorsal and lateral 

 egions. The above peculiarities of the ectoderm at and immediately 

 iu front of the growing zone are however rendered partly intelligible 

 by this comparison. 



The space between eutoderm and ectoderm at the growing zone 

 is filled with a mass of undiflerentiated mesoderm; more anteriorly 

 the mesoderm beeomes differentiated and shows the usuai division 

 luto somites. At the extreme posterior uarrow region between the 

 entoderm and the ventral ectoderm are found undiflerentiated cells 

 of a characteristic embryonic appearance (Piate 23, Figs. 10, 30, 31). 

 These cells are typìcally larger and clearer than those situated 

 more anteriorly, and are frequently found in process of mitosis; 



bey possess large clear nuclei each of which typically contains a 

 conspicuous vacuolated nucleolus. They undoubtedly correspond to 

 the teloblasts or mesoblastic pole -cells which terminate the meso- 

 derm bands in so mauy other species. In Arenicola, however, their 

 uumber is not Constant, nor are they sharply defined from the meso- 



blast cells immediately iu front. The extreme posterior mesoderm 



