284 WATASE. [Vol. IV. 



come of them meet with the rays of the opposite side in the 

 plane of the equatorial chromatin "plate." This plane corre- 

 sponds to the median axis of the ovum. 



Fig. 1 8 represents the blastoderm with the first furrow of 

 cleavage completed. The furrow is deepest in the area where 

 the layer of cytoplasm is thickest, becoming gradually shallower 

 and shallower until it finally disappears in the peripheral zone of 

 the blastoderm, or in the zone of the protoplasmic pellicle. 

 Each daughter nucleus has a characteristic bean-shaped outline, 

 with the broader convex borders turned toward each other. 

 The outline of the original interzonal substance is not distinct. 



The direction of the furrow of cleavage corresponds exactly 

 with the plane of the median axis of the adult organism. The 

 two segments of the blastoderm correspond to the right and the 

 left half of the adult organism respectively. The " median " 

 or the " unpaired " structure as such, therefore, strictly speak- 

 ing, has no existence in the squid. All parts are paired histologi- 

 cally, and derive their material from two bilateral sources. The 

 median structure, the siphon, arises in that way, as is well known. 

 Organs like the digestive tube, must also be considered as arising 

 by the meeting of some descendant of cells derived from two 

 halves of the original blastoderm such as I have been describing. 



Fig. 19 represents a stage in which each of the two daughter 

 nuclei of the previous figure is dividing. In the nature of the 

 archoplasmic spindle and the appearance of the equatorial 

 chromosomes, they do not offer any recognizable difference 

 from those seen in a preceding stage. One important point to 

 be noticed in this stage is the direction of the longitudinal axis 

 of the caryokinetic figure in reference to the plane of the first 

 cleavage furrow. The axis of the spindle does not run exactly 

 in the same direction with the first cleavage furrow, but slightly 

 diverges from it anteriorly. If the axes of the two spindles on 

 both sides of the first cleavage furrow be prolonged both ante- 

 riorly and posteriorly, they will soon meet in the prolongation 

 of the median plane of the blastoderm in the posterior part, 

 while they will diverge more and more as they are prolonged 

 in the anterior part. 



This stage introduces us into the next one (Fig. 20), where 

 the slight antero-posterior differentiations shown in the disposi- 

 tion of the caryokinetic figure in relation to a fixed plane in the 



