54. lilOHAKD EVANS. 



the ectoderm^ and decrease in size from in front backwards. 

 The common rudiment of the nervous system and ventral 

 organs has been separated ivom that of the appendages, and 

 in the region of the third and fourth somites the thickening 

 in question has been divided into two, namely, an internal 

 one, the primordium of the nervous system, and an external 

 one, the forecast of the ventral organs. The ectodermal 

 thickenings are continuous from the cephalic lobes backwards, 

 —that is, the rudiment of the para-cesophageal cord is already 

 formed. 



Owing to the way in which they develop from in front 

 backwards, the mesodermal somites in the embryo under 

 consideration illustrate in the most perfect manner the 

 chauges through which they pass in their development up to 

 a certain stage, a result made possible by the most admirable 

 series of sections into which the embryo was cut. But the 

 general remarks I have to make on these will be reserved for 

 another section of the paper, in which the coelom and the 

 mesodermal organs will be specially considered. At present 

 it suffices to say that there are twenty-seven pairs of them, 

 and that it is impossible to state with any degree of certainty 

 whether one pair moi-e would have been developed or iiot. 

 There are no traces whatever of the germinal nuclei, either 

 in the mesoderm or endoderm. 



In the embryo under consideration the endoderm is very 

 different from what it was in the second embryo described. 

 It is no longer possible to speak of a peripheral layer of endo- 

 derm containing nuclei, and a central mass of food yolk devoid 

 of nuclei. It would be preferable to speak of a peripheral 

 Liyer and central mass of endoderm ; for the endodermal 

 elements of the peripheral layer of the previous stage have 

 invaded the central mass and converted the central yolk 

 into a number of nucleated masses, which present the same 

 structure as the peripheral layer did in the previous stage- 

 In both the peripheral layer and the central mass spherical 

 masses of stored-up food materials, and almost innumerable 

 "granules small and refringent in character, are present; 



