CRANGON VULGARIS. 129 



these eventually become partly mesodermal and partly en- 

 dodermal. Metschnikoflfs slight account of the early 

 stages of Geophilus would indicate that the Chilopods are 

 much like the Diplopods. Sograf ('83) has a Russian 

 paper on Geophilus from which it would appear that there 

 is a similar migration of some eel Is to form the blastoderm 

 while others remain behind in the yolk. 1 The mesoderm 

 arises in much the same way, but some of the later sections 

 would tend to show that not all the nuclei remaining behind 

 in the yolk were utilized in forming the endodermic epithe- 

 lium but that they were utilized as food like the yolk. 

 One figure would seem to indicate that the endoderm may 

 bud off cells to take a place among the mesoderm. 



This confusion regarding the origin of the endoderm in 

 the hexapods, arachnids and inyriapods, and the belief that 

 the facts shown by the decapods aid in an interpretation 

 of the various phenomena are my excuse for thus taking 

 up more space than, perhaps, the subject demands. The 

 existence of a gastrula stage in all Metazoa, whether of 

 the type of " archigastrula " or of some of the numerous 

 modifications recognized by Haeckel, is admitted by all; 

 but, so far as I am aware, no one has as yet brought the 

 hexapods in full accordance with that theory. My present 

 attempt may not be deemed more satisfactory than the 

 twenty or more that have preceded it ; it has, however, 

 the merit of reconciling more facts than any other. 



1 In the abstract quoted from (see bibliography) it is stated that the eggs of 

 myriapods are very peculiar in that they have the protoplasm at first at the centre, 

 the cells migrating to the surface, and it is suggested that this probably distin- 

 guishes them from the Arthropoda ' ; since in no other arthropodous form does the 

 vitellus so constantly occupy a superficial position and so completely invest the 

 first segmentation cells, which are then aggregated in a cluster at the centre of 

 the egg" 1 It is one endeavor of the present paper to show that just this condition 

 is characteristic of the Arthropoda as a whole; and (Tetranychus and the mero- 

 blastic forms excepted), so far as the writer is aware, there is not a single arthropod 

 in which there is an abundance of food yolk but what has just this type of egg, 

 here regarded as decidedly myriapodous. 



