BIGELOW: EARLY DEVELOPMENT OF LEPAS. 13 
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 or Literature on Late Staces or Cieavace, on Cuos- 
ING OF THE BLASTOPORE, AND ON DIFFERENTIATION OF 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 in 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 that 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, but 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 which 
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-ento- 
blast. Groom’s statement (p. 198) that epiblastic cells continue to be 
