CLEAVAGE AND FORMATION OF THE BLASTODERM. 115 



(b) The blastoderm develops first on the ventral side of the egg. 

 Its formation begins at one point on the surface and proceeds 

 gradually from this point, which always represents the future ventral 

 side of the egg; in the Decapoda the point denotes the most posterior 

 end of the ventral side, or the spot at which, later, the gastrula 

 invagination appears. This has been observed in Palaemon and 

 Eriphia, in which the formation of the blastoderm is completed 

 over the whole surface of the egg and closed on the dorsal side, only 

 when the rudiments of the embryo are already to be seen on the 

 ventral side. 



These modifications in the formation of the blastoderm are found both in 

 Type II. and Type III., and thus result in four subsidiary methods of cleavage, 

 which deserve detailed description : 



Type II. a. The cleavage is at first total, and later superficial, being followed 

 by the simultaneous formation of the blastoderm all over the surface ; Sranchipus, 

 free-living Copepoda, summer eggs of Polyphemus and Bythotrephes, Eupagurus. 



Type II. b. The cleavage is at first total, later superficial, the blastoderm 

 developing first on the ventral side. This method is exceedingly common 

 among the Amphipoda. As, in this method, the cells on the future ventral side 

 divide more rapidly, and there is in this region an early separation of the 

 blastoderm-cells from the food-yolk, a considerable difference is found between 

 the cells of the ventral side and the large cells of the future dorsal side which 

 abound in yolk. It is evident that this type of cleavage, which is seen in 

 various Amphipoda (especially in Gammarus locusta, according to VAN BENEDEN 

 and BESSELS, No. 2), greatly resembles in its first stages total, unequal cleavage. 

 An important distinction, however, arises from the fact that in this case the 

 pole at which the small cells are found belongs to the vegetative half, .that 

 portion of the surface at which, later, the formation of entoderm takes place, 

 while the larger cells belong more to the animal region (the later dorsal side). 

 In any case the two axes here compared (that passing through the animal and 

 vegetative poles, and that from the small-celled to the large-celled pole) do not 

 appear to coincide, but to cross each other obliquely (cf. below, p. 142). 



Type III. a. The cleavage is purely superficial, the subsequent formation of 

 blastoderm taking place all over the surface simultaneously ; many Decapoda 

 \Penaeus, Astacus, Callianassa), all the winter eggs and many summer eggs 

 of the Cladocera. 



Type Ill.b. The cleavage is purely superficial, the blastoderm developing 

 first on the ventral side of the egg. In this case a few of the numerous 

 elements to be found within the yolk first shift to a definite point at the surface, 

 there to be transformed into blastoderm-cells. There thus arises a small 

 blastoderm-disc, corresponding in position to the future ventral side- .of the egg ; 

 this disc gradually increases in size, new elements from within the yolk con- 

 stantly rising to its periphery and becoming changed into blastoderm-cells. 

 This method of cleavage consequently strongly resembles the discoidal cleavage 

 to be described presently. The distinction between the two types consists in 

 the fact that in Type III. b, the increase of the blastoderm is due to the accession 

 of new elements from the inner portion of. the egg, while in true discoidal 

 cleavage (IV.), the increase takes place exclusively by the .division of the elements 



