OF THE C.EKMINAL LAYERS IN GASTROPODA. 7 



vegetative poles. It stands almost at right angles to tlie first 

 cleavage plane and coincides with the future median axis of the 

 body. As a consequense the egg is subdivided into the right, 

 and the left halves (Fig. 7). Concerning the priority of the 

 cleavage in the first two blastomeres just described, there is much 

 controversy. In Umbrella, Aplysia, and others the posterior 

 blastomere divides first. On the contrary, in Neritma, Unio and 

 others the anterior precedes the other. Thus it is easy to see 

 that in Mollusca n eggs the priority of cleavage is quite independent 

 of the amount of the deutoplasm and its distribution in the egg. 

 The four cells formed by the second cleavage take a spherical 

 form just as i'l the case of the first blastomeres, but soon become 

 flnttcr.^d on one side as the result of mutual contact. They, as 

 a whole, constitute the first quartet and are commonly designated 

 as A, B, C, and D, (Fig. 8) of which A and B come from the 

 anterior, and C and D from the posterior, blastomeres of the 

 two cell stage. During the formation, and especially during the 

 accommodation of this quartet the cell C shifts its position 

 obliquely upwards. At length, it comes to stand in the same 

 level with the two anterior blastomeres leaving its complement 

 cell J) in the original level lower than the others. By this shifting 

 it also happens that the two diagonally opposite cells B and D 

 are brought in direct contact at the animal, as well as at the 

 vegetative, poles, forming what has been called the cross furrow. 

 The cells forming this furrow differ in different species, and 

 sometimes the furrow is formed by different cells at the two poles. 

 Moreover, there are many cases, in w^hich the length and the 

 direction of the furrow presents marked differences in these two 

 poles. However, in the species under consideration, the furrow 

 is formed by the juxtaposition of the same cells both in the 



