IV 



DEVELOPMENT PREVIOUS TO THE SWIMMING STAGE 



Oosperm and Segmentation Stages. — As soon as fertilization com- 

 mences the egg takes on a more spherical shape, but the o5sperm, one or 

 two hours later, lengthens again in the same direction as is common 

 among fresh unfertilized eggs. Oriented in the position in which the 

 heavier yolk-bearing portion is below and the more protoplasmic portion 

 carrying the polar bodies above (Plate I, fig. 2; Plate V, fig. 10), the 

 oosperm possesses polarity, the lower pole being especially nutritive (vege- 

 tative, vitelline), the upper formative (animal, protoplasmic). The 

 more unwieldy yolk has a retarding influence, which finds expression in 

 the very first efforts at segmentation. Instead of there being an equal 

 division into two completely symmetrical hemispheres, the first cleavage 

 affects directly only that portion which belongs to the formative pole 

 (Plate I, fig. 3; Plate V, fig. 11), with the result that two almost equal 

 blastomeres are formed, below which is suspended an undivided mass, con- 

 taining most of the deutoplasm, adhering rather more closely to the larger 

 blastomere. This mass itself resembles a blastomere and may be con- 

 veniently spoken of as the deutomere. The constricting furrow deepens 

 until the figure presents three rounded lobes in one plane and meeting at 

 one point, while the polar globules are retained in the furrow between the 

 two smaller lobes above. After the first energetic efforts at division the 

 furrows relax until the larger of the two blastomeres completely re-unites 

 with the deutomere (Plate V, fig. 12), the whole remaining for a time in 

 a resting condition (resting period). The figure then presents the appear- 

 ance of a small blastomere (micromere) budded off from a large one 

 (macromere). 



The second cleavage likewise affects only the formative pole, dividing 

 the two blastomeres — which re-emerge — through the insertion of the 

 polar bodies, in a direction at right angles to the first cleavage (Plate I, 

 fig. 4; Plate V, fig. 13). One of the halves of the larger blastomere again 

 retains more intimate connection with the deutomere, giving the latter 

 a slightly excentric position. The blastomeres can now no longer be 

 brought into a single plane with the deutomere, but form a horizontal 

 plane of their own, and the only vertical plane that can divide the whole 

 into equal halves falls through the deutomere, the blastomere above it, 

 and the middle one of the other three, most clearly seen in the resting 

 period which follows. 



