ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



that with a few possible exceptions, which arc by no means well established, the 

 polar bodies arc formed at the animal pole of the egg in all cases. This is a fact of 

 the most general occurrence and of the highest significance : it indicates that before 

 or during the maturation of the egg there occurs a polar differentiation or localiza- 

 tion of the egg substance of such a kind that in all cases the future ectoderm is 

 formed at the maturation pole and the endoderm at the opposite pole. 



The apparent exceptions to this rule are few in number and may be examined 

 in some detail ; they are limited to the eggs of certain insects, Petromyzon, 

 copepods, Ascaris, echinoderms and aseidians. The only reason for supposing, as 

 Korschelt and Heider (1903, pp. 545, 546) do, that the polar bodies are not formed 

 at the animal pole in insects and in Petromyzon is that they here lie to one side of 

 the pointed end of the egg; there is no proof that they do not lie at the middle 

 of the ectodermal area. Hacker (1899) says that in the larger species of Cyclops 

 "neither the place of formation of the polar bodies, the place of entrance of the 

 sperm nor the position of the first cleavage spindle are preformed in the egg, but 

 are secondarily determined by the position of the egg in the egg sack" (pp. 

 193, 194). However, this egg is one which is not easy to orient, and it has by no 

 means been proven that the polar bodies do not form in this case at the middle of 

 the ectodermal area. Even if the justice of all of Hacker's statements bo admitted 

 it has not been shown that the cleavage spindle may not rotate so as to cause the 

 first and second cleavage furrows to pass through the maturation pole, as is usually 

 the case. Such a rotation of the first cleavage spindle takes place in nematodes, 

 and a somewhat similar rotation of the entire egg, after the formation of the first 

 cleavage spindle, has been described by Bigelow (1902) in the case of Lepas, where 

 it had previously been held that the first cleavage was equatorial. Hacker's obser- 

 vations do not show that the chief axis of the egg is not predetermined, and they 

 certainly do not prove that the maturation pole and the ectodermal pole do not 

 coincide. 



In Ascaris megalocephala, Boveri (1887) observed that the second polar body 

 is usually formed at some distance from the first "whether through wandering in 

 the protoplasm or through a turning of the entire egg I could not determine" (p. 

 32). His figures (1888, pi. IV) show that the first cleavage furrow frequently 

 passes through the point of attachment of the second polar body. The study of 

 the cell lineage of Ascaris has shown that most of the ectoderm is segregated in 

 one of the first two cleavage cells (the "primary ectoderm cell" of Zur Strassen, 

 1896). This would seem to indicate that in this animal the polar bodies-do not lie 

 at tin' middle of the ectodermal pole; however the relations of the maturation 

 pole to the ectodermal pole and to the first cleavage arc not clear in this case, and 

 it may not be impossible that Ascaris may yet be found to conform to the general 



rule. 



As for the echinoderms. Wilson (1895) supposed from indirect evidence that 

 the maturation pole and the future animal pole did not usually coincide in 

 To.ropneustes, and further that the chief axis of the egg was established only after 



