BIGELOW: EARLY DEVELOPMENT OF LEPAS. 99 



oriented. The bilateral arrangement of cells when the egg is viewed 

 from the animal pole and the position of the yolk near the vegetative 

 pole (Figs. 38, 66) are features which aid in quickly identifying the 

 individual cells when the egg is rolled into proper positions. 



During the third cleavage the polar cell is usually crowded beneath 

 the blastoderm, and comes to occupy in the cleavage cavity the position 

 indicated in Figure 66 — a condition which has been described as occur- 

 ring in the eggs of several other Eutomostraca. Sometimes at the close 

 of this cleavage it is found lodged between cells. Occasionally it be- 

 comes shifted in the earlier stages so that it no longer lies deep in the 

 cleavage furrow ; in such an event it is not forced beneath the blasto- 

 derm during the third cleavage, but may be found on the surface in 

 later stages. I have noticed it on the outside of the embryo in stages 

 as late as those of about five hundred cells. In such cases it is some- 

 times far from its normal position at the anterior dorsal side (animal 

 pole) of the embryo. In its usual position beneath the blastoderm the 

 polar cell is quite definitely situated until very late stages. In the 

 eight-cell stage it is almost equidistant from the two poles of the chief 

 axis of the egg; but it usually lies much nearer the animal pole after 

 the fourth cleavage, and is a very nseful *' landmark " for orientation of 

 the later stages. In good transparent preparations of entire eggs of any 

 cleavage stage the polar cell is clearly visible, and it is often seen lying 

 beneath the blastoderm in stages with over five hundred cells. 



The yolk-cell of the eight-cell stage (d*-\ Plate 5, Fig. 40 ; Plate 8, 

 Fig. 66) contains onl}' future mesoblast and entoblast, and will be re- 

 ferred to as mes-entoblast. The third micromere (d*-^), separated 

 from the yolk-cell in the third cleavage, is purely ectoblastic, and is the 

 last cell containing ectoblast which is given off* from the yolk-macro- 

 mere. The ectoblast is, therefore, separated from the yolk-laden ento- 

 blast in the fii'st three cleavages, being contained in the derivatives of 

 tlie three micromeres, aP, c^ and d*-"^, which are separated from the yolk- 

 l)earing macromere in the first, second and third cleavages respect- 

 ively. A study of the cell-lineage through the later stages of cleavage 

 shows that the cells ab^ and c^ are not purely ectoblastic, but contain 

 a portion of the future mesoblast ; they may, therefore, be called mes- 

 cctoblasts. Of their descendants in the eight-cell stage, the cells at the 

 animal pole (a^*^ b*-% c*"^) are purely ectoblastic, while the lower cells 

 around the vegetative pole (a*'^ i**S c*'^) contain future "secondary 

 mesoblast" (ectoblastic mesoblast). 



