28o WATASE. [Vol. IV. 



be said to be an aberrant feature unrepresented in the ancestral 

 protozoa, so long as the existing forms of the protozoa often 

 show such a high degree of differentiation in that particular 

 respect. 



It appears to me admissible to say at present that the ovum, 

 which may start out without any definite axis at first, may 

 acquire it later, and at the moment ready for its cleavage the 

 distribution of its protoplasmic substances may be such as to 

 exhibit a perfect symmetry, and the furrows of cleavage may 

 have a certain definite relation to the inherent arrangement of 

 the protoplasmic substances which constitute the ovum. Hence, 

 in a certain case, the plane of the first cleavage furrow may 

 coincide with the plane of the median axis of the embryo, and 

 the sundering of the protoplasmic material may take place into 

 right and left, according to the pre-existing organization of the 

 egg at the time of cleavage ; and in another case the first cleav- 

 age may roughly correspond to the differentiation of the ecto- 

 derm and the endoderm, also according to the pre-organized 

 constitution of the protoplasmic materials of the ovum. 



It does not appear strange, therefore, that we may detect a 

 certain structural differentiation in the unsegmented ovum, with 

 all the axes of the future organism already foreshadowed in it, 

 and the axial symmetry of the embryonic organism identical 

 with that of the adult. 



The general shape of the ovum of the squid at the time it is 

 ready for cleavage is oblong, having the shape of a hen's Qgg, 

 with one end more pointed than the other (Fig. i, PI. IX). On 

 the pointed end is situated the germ-protoplasm, spreading its 

 thin pellicle over the huge mass of the food-yolk and completely 

 enveloping it. The germ-protoplasm at the pointed pole shows 

 such a thickening, that, when viewed from the side, it presents 

 a lenticular outline, sharply distinguished from the underlying 

 mass of food-yolk {Fig. X). The polar globules, three in num- 

 ber, one a little larger than the rest, are seen floating in the 

 perivitelline fluid inside the chorion. 



By keeping the exact position of the segmentation nucleus 

 and the outline of the whole germ-disc as shown by the optical 

 section in view, and rolling the egg in its longitudinal axis, we 

 observe two important facts. At one time we observe a slight 

 inequality in the amount of cytoplasm on both sides of the 



