THE DEVELOPMENT OF ADMETUS I'UMILTO. 599 



a batch of eggs which are all at about the same stage of 

 development. 



Surface views of these eggs show that the blastoderm is 

 already formed. It appears as a long white ribbon covering 

 about one third of the surface of the egg in length and one 

 sixth in breadth. At one end the white patch seems much 

 thicker, being more opaque in reflected light. The blasto- 

 derm is not of the same breadth throughout its length, but 

 is somewhat nai'rower in the middle, and rounds out at both 

 ends. The margins are not distinct or abrupt, but gradually 

 fade away until at last they become invisible. 



It is not possible to distinguish any further details super- 

 ficially, the nlaterial having, unfortunately, been kept too 

 long in alcohol. 



Sections through the egg at this stage show that the 

 blastoderm already consists of the thi-ee germinal layers. 



The ectoderm consists of a continuous epithelium or layer 

 of cells covering the surface of the egg. The Avails of 

 these cells are distinctly visible. The cells themselves are 

 rounded, their nuclei fairly large and somewhat oval ; they 

 measure '01 mm. in length by "007 mm. in breadth. The 

 chromatin is r,ot evenly distributed, but foruiiiig a number of 

 small particles, gives the nuclei a spotted appearance. This 

 hiyer is sometimes as much as three cells deep. 



The mesoderm lies directly under the ectoderm. The cells 

 forming it arc disconnected ; whether they form a continuous 

 layer in life or not I am not able to decide, as the cells may 

 have shrunk in alcohol. Between the cells of the mesoderm 

 yolk particles are often to be seen, which makes it probable 

 that the mesoderm never consisted of a continuous layer. 

 The nuclei of the mesoderm cells are larger than those of 

 the ectoderm, and also rounder ; they measure '014 mm. in 

 diameter. 



The cells of the mesoderm seem to be engaged in rapid 

 reproduction, karyokinetic phenomena being very frequent, 

 and the centrisomata very often visible. 



The entoderm consists of single cells, lying deeply 



