BIGELOW: EARLY DEVELOPMENT OF LEPAS. 113 



Summary of the Seventh Cleavage. 



All cells, except the two entoblasts, divide. 



Derivatives of the two kinds of mesoblast have not been distinguished 

 after the cells are crowded together at the posterior end. 



There is no evidence that mesoblast originates otherwise than as de- 

 scribed in the preceding account of the sixth cleavage. The entoblast 

 nuclei have been traced from the sixteen-cell stage and there has been 

 but one division. Hence, contrary to the assumption of earlier investi- 

 gators, the entoblast nuclei cannot contribute to the mesoblast (see the 

 following review of the literature). 



11. Review of Literature on Late Stages of Cleavage, ox Clos- 

 ing OF the Blastopore, and on Differentiation op the Germ- 

 Layers. 



a. Late Cleavage. — Groom ('94) did not follow the later cleavages in 

 detail, because his results showed so great variation in the early stages. 

 He describes the later growth of the blastoderm over the yolk as " tak- 

 ing place in precisely the same manner as iu the earlier stages, i. e., by 

 the emergence of merocytes from the yolk and the division of blasto- 

 derm cells. . . . The variation is so great tliat the process may be said 

 to be irregular. ... I am unable to say how many merocytes take part 

 in the formation of the blastoderm ; but in all probability the number 

 is variable, V)ut not large. As the ovum is often half covered when 

 four or five have emerged, some such number as nine or ten may not be 

 far from the mark" (Groom, '94, pp. 140, 141). 



The supposed variation in early stages of cleavage has already been 

 discussed in the reviews of the literature on those stages. The later 

 cleavage and growth of the blastoderm have been shown in this paper 

 to be very regular, and the variations upon which Groom has placed 

 much stress are comparatively rare. These variations can usually be 

 ascribed with strong probability to unfavorable conditions in the en- 

 vironment of the developing egg. The number of " protoplasmic " cells 

 (micromeres) formed from the yolk-cell has been shown to be not varia- 

 ble (nine or ten), as Groom supposed, but constant, viz. four, of wliich 

 the first three — containing all the ectoblast and " secondary mesoblast " 

 — are separated from the yolk by the first three cleavages, while the 

 fourth cleavage differentiates the primary mesoblast from the yolk-en to- 

 blast. Groom's statement (p. 198) that epiblastic cells continue to be 



