226 F. G. HEATHCOTE. 



tion of their long axis ; and, in fact, before they do begin to 

 take part in the formation of the mesodermic keel, they undergo 

 an alteration, which I shall describe. When first the endo- 

 derm cells just mentioned begin to come together in the middle 

 line near the ectoderm their appearance is somewhat peculiar; 

 their nucleus is small, round, and deeply stained ; their form is 

 stellate and their outline very distinct. 



Processes pass from them to the ectoderm cells. This is 

 shown in fig. 19, which is a transverse section through an ovum 

 on the fourth day, taken in a plane such as to cut through the 

 first beginning of the keel. When a fair number of these cells 

 are assembled in the middle ventral line a change takes place 

 in the cells of the ectoderm just outside them. The latter 

 become more rounded, while their nuclei, instead of being long 

 and ova], become round. In fact they undergo an alteration 

 which causes them to resemble the cells which I have described 

 as assembling immediately below them. This alteration is 

 shown in figs. 19 and 20, which are transverse sections through 

 the first beginning of the keel. 



The ectoderm cells in the middle line, after altering their 

 shape as I have described, increase by division, and take a con- 

 siderable share in the formation of the keel. The cells in the 

 middle line, both ectoderm and endoderm, continue to increase, 

 and are joined by more cells from the hypoderm, and eventually 

 on the fifth day we find a keel in the middle ventral line, 

 something like that described by Balfour in his paper on the 

 development of Agelena labyrinthica (16). Both ecto- 

 derm and endoderm have taken part in the formation of the 

 keel. 



When the keel is fully formed the cells of which it is com- 

 posed are large, somewhat irregular in shape, and have a large 

 nucleus. They are all directly connected together, though, 

 owing to their being closely packed together, it is difficult to 

 see anything of their connections, except where one cell has 

 been somewhat separated from the others. The keel is of con- 

 siderable thickness, being about six or more cells deep in its 

 thickest part. 



