ö INTRODUCTION 



way a disc-shaped embryonal part which is situated on the 

 unsegmented yolk-mass at the animal pole. We call this 

 type of segmentation, which repi'esents the most extreme 

 case of unequal cleavage, discoidal cleavage. It occurs, for 

 example, in the Cephalopods. 



A particular type of cleavage, which does not fit into the 

 above series, occurs in the class of Arthi-opods. Whereas 

 all eggs thus far considered were characterized by a more 

 or less considerable accumulation of yolk in the region of 

 the vegetative half (telolecithal eggs), the distribution of 

 the yolk being accordingly eccentric, the eggs of Arthro- 

 pods exhibit a regular disti-ibution of the yolk masses of 

 such nature that their centre coincides with the middle 

 point of the egg (centrolecithal eggs). The first cleavage 

 nucleus here lies in the centre of the egg, where by divi- 

 sion it separates into a large number of nuclei, which are 

 distributed uniformly at the periphery of the egg, and thus 

 give rise to the formation there of a layer of small uniform 

 blastomeres. This cell-layer represents the blastoderm, 

 while the cleavage cavity of the blastula-stage produced in 

 this way is filled with the unsegmented yolk-mass. This 

 kind of cleavage is known as superficial cleavage. 



The modifications of development hitherto considered ap- 

 pear to be dependent principally upon the amount and mode 

 of distribution of the yolk matter. We have still to consider 

 some forms which in the mode of distribution of the yolk 

 recall the centrolecithal eggs, but which by their peculiar 

 mode of entoderm formation prove to be aberrant forms. In 

 the first place, there should be mentioned in this connection 

 the kind of entoderm formation (by delamination) occurring in 

 the Cnidaria (Hydroids). The typical case of this kind 

 exists in the development of the GeryonidjB. A coeloblastula 

 is here formed by total and equal cleavage, and there follows 

 a division of the cells in such a manner that an inner portion 

 rich in yolk becomes separated fi'om a superficial part with 

 little yolk matter. In this way there arises out of the one- 

 layer sphere an arrangement of the cells into two concentric 

 hollow spheres, of which the inner contains the elements of 

 the entoderm, and the outer those of the ectoderm. One 



