226 bulletin: museum of compakative zoology. 



Summary on Polarity of the Egg. 



(1) 77/e dorso-ventral axis of the embryo is predetermined in the egg 

 before fertilization ; the polar globules invariably form at its dorsal pole. 



(2) The spermatozoon may enter the egg at any point on its ventral 

 hemisphere., that point probably determining, however., the tnedian plane 

 and posterior end of the embryo. 



(3) If we adopt the couimouly emploj'ed 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 protojjlasm, 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 OP 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 nomenclature to be 

 employed. 



1. Nomenclature. 



In any extended work on cell lineage it is desirable to have some 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 



