ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. Ill 



Relatively slight modifications of this localization, however produced, may lead 

 to profound modifications of the resulting embryo and adult. I have elsewhere 

 (1903) shown reason for believing that the cause of inverse symmetry is to be 

 found in the inverse organization of the egg, and that this inverse organization may 

 pjssibly be produced by the maturation of the egg at opposite poles in dextral and 

 sinistral forms. This ease shows that one of the most remarkable forms of muta- 

 tion with which we are acquainted may be the result of modifications in the localiza- 

 tion of germinal substances in the unsegmented egg. 



One of the great difficulties in explaining the origin, on evolutionary princi- 

 ples, of different phyla has been the dissimilar locations of corresponding organs or 

 parts. These difficulties are well illustrated by the theories which attempt to 

 derive the vertebrates from annelids, or from any other invertebrate type. Without 

 assuming to defend any of these theories it may here be pointed out that if evolu- 

 tion takes place through modifications of germinal organization, it is no more 

 difficult to explain the different location of parts than their different qualities. 

 Changes in the relative positions of parts, which would be impossible in the adult, 

 may be readily accomplished in the unsegmented egg, as is shown by cases of inverse 

 symmetry. The question is here raised, whether some similar sudden alteration of 

 germinal organization may not lie at the basis of the origin of new types. 



SUMMARY. 

 I. Ascidian Embryology. 



1. The orientation of the ascidian egg and embryo adopted by Van Beneden 

 and Julin is correct, that of Seeliger, Samassa and Castle is wrong (pp. 20-37). 



2. The cell-lineage given by Castle is correct for the early stages; from the 

 48-cell stage on it is wrong (pp. 5 65 U ) . 



3. The egg axis corresponds very nearly with the gastrular axis; during the 

 closure of the blastopore this axis is shifted so that it is no longer dorso-ventral as 

 in the early stages, but is antero-ventral and postero-dorsal in direction in the larva 

 (pp. 73, 75-77). 



4. The relation of the germinal layers to the cleavage planes is very different 

 from the account given by Van Beneden and Julin and by Castle, and is more 

 nearly in accord with that of Seeliger, Davidoff and Samassa. All cells above the 

 equator (3d cleavage plane) are ectodermal and neural plate cells; all below are 

 endodermal, mesodermal and neural plate (pp. 47-48). 



''5. The factors of gastrulation are (a) change of shape of the cells of the 

 animal and vegetal hemispheres, (b) overgrowth of the marginal cells (pp. 64-65). 

 Peculiarities of the gastrula are foreshadowed in the egg at a very early stage 

 (pp. 45. 50. 59). 



/ 6. The muscle and mesenchyme cells arise from a common base, the meso- 

 dermal crescent, which surrounds the posterior side of the egg just below the 

 equator (pp. 19-21); ultimately these cells surround the posterior margin of the 



