34 PROCEEDINGS OF THE AMERICAN ACADEMY 



cleavage-groove, and a more or less distinct constriction between the 

 margin of the disc and tlie yolk. 



In Nassa the two blastomeres succeed in attaining a completely 

 spherical form and the equatorial constriction is carried to the point 

 of separation. As soon, however, as the meridian cleavage is finished, 

 one of the blastomeres coalesces with the deutoplusmic mass, and tlie 

 result is two very unequal cleavage-spheres. The second cleavage- 

 plane, which is also a meridian plane, at right angles to the first, is 

 accompanied by the same equatorial constriction, and followed by a 

 coalescence which completely cancels the effect of the constriction, 

 leaving a stage of four blastomeres. Leaving these constrictions out 

 of account, as forming no part of the proper cleavage, we may saj' 

 that the ovum of Nassa follows the general law of cleavage, in be- 

 ginning with two meridian grooves. 



According to Brooks,^^ the early stages of cleavage in the oyster 

 are identical with those in Nassa. The *' exceptions to the normal 

 metliod of segmentation " plainly demonstrate tliat the trefoil and the 

 cinquefoil stages represent, in reality, the 2-cell and the 4-cell stages. 



The fact, if it be a fact, that the axis of the ovum is homolo- 

 gous in the higher animals, implies much more than has yet been 

 stated. It implies that certain definite and fundamental relations are 

 predetermined in the unsegmented ovum, among which may he men. 

 tioned constant relations, first, between the poles and the germ-layers ; 

 second, between the axis of the ovum and the axis of the embryo ; and 

 third, between both these axes and the first cleavage-planes. For 

 reasons which do not require to be stated here, there is a much higher 

 probability that the fir.st one or two cleavage-planes sustain uniform 

 relations witli the axis of the ovum, than that the later planes do so. 

 We cannot here enter into a discussion of the general bearinfj of these 

 several points ; but one of these, viz. the relation of the first cleavage- 

 plane to the axis of the ovum, deserves something more than a passing 

 notice in this connection. It is somewhat surprising, in view of the 

 facts now before las, that recent writers have gone on talking about 

 cleavage as if it were nothing extraordinary for it to begin with an 

 equatorial or parallel groove. It is plainly a matter of fundamental 

 importance, especially in ova with a pronounced polar segregation of 

 material, wiiether the first cleavage-plane passes through the ]io]e 

 from which the polar globules issue, or at right angles to the axis 



81 Brooks. " Development of the Oyster." Studies from the Biolog. Lab., 

 I., No. i, 1880. 



