226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Summary on Polarity of the Egg. 
(1) The dorso-ventral axis of the embryo is predetermined in the egg 
before fertilization; the polar globules invariably form at its dorsal pole. 
(2) The spermatozoón may enter the egg at any point on its ventral 
hemisphere, that point probably determining, however, the median plane 
and posterior end of the embryo. 
(3) If we adopt the commonly employed terms animal and vegetative 
for the two poles of the unfertilized egg, we must call the ventral the 
animal pole, and the dorsal the vegetative pole. For it is the ventral 
half of the egg which contains a richer supply of protoplasm, and which 
consequently cleaves more rapidly and becomes the ectodermal side of 
the embryo; whereas the dorsal half of the egg contains less protoplasm, 
cleaves less rapidly, and forms the endodermal portion of the embryo. 
(4) We may say, accordingly, that the form changes accompanying ma- 
turation occur, in Ciona at least, and presumably in Ascidians in general, 
at the pole of the egg opposite to that at which they occur in Amphioxus, 
and, so far as known, in all other animals producing eggs with polar 
differentiation ; for the changes connected with maturation are uniformly 
reported to take place at the animal, i. e. at the more richly protoplas- 
mic pole, whereas in Ciona they take place at the vegetative pole. 
VI. CELL LINEAGE OF THE EMBRYO. 
The statement made in the preceding paragraph presents a condition 
of affairs so directly contrary to that found in other groups of animals, 
as well as to what has been assumed by all previous writers to be the 
case in Ascidians, that it requires the presentation of unmistakable 
evidence in its support. Such evidence I have to offer, both from 
the study of the living egg and from that of preparations. Before 
passing, however, to the consideration of this evidence, a word of 
explanation is necessary concerning the system of nomenolature to be 
employed. 
1. Nomenclature. 
In any extended work on cell lineage it is desirable to have somo sys- 
tem of naming the individual cells which will indicate readily the exact 
history of each, — from what part of the matured ovum it has been 
derived, by how many divisions it is removed from the ovum, and from 
what other cells these divisions have separated it. In this paper I shall 
