84 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



In Amphioxus the earliest stage at which the chorda has been positively iden- 

 tified is one when the blastopore is small and the embryo elongated. According to 

 Hatschek it consists at this stage of a plate, about six cells wide, in the roof of the 

 archenteron and extending along the mid-line of the dorsal lip throughout its entire 

 length. This plate is narrower and longer than it is in the ascidians. but is other- 

 wise much the same in appearance. The later history of the chorda is essentially 

 the same in both forms. With regard to the origin of the chorda cells in 

 Amphioxus, Morgan and Hazen (1900) have shown that the cells which are inrolled 

 in the formation of the dorsal lip and some of which must take part in the forma- 

 ation of the chorda, are clear and contain less yolk than the endoderm cells. 

 Whether these cells form at this stage a plate which is wider from side to side 

 than it is long, as is true of the ascidian, is not known. Lwoff (1804) has also 

 recognized the fact that the chorda cells are rolled in at the margin of the dorsal lip, 

 and for that reason he regards them as of ectodermal origin. 



In Amphioxus and in some amphibians the definitive roof of the enteron arises 

 from cells which lie along each side of the chorda plate, and which finally grow 

 under that structure and thus separate it from the gastric cavity ; in the ascidians the 

 chorda lies ultimately in the posterior part of the body where the gastric cavity is 

 almost entirely lacking and there is no growth of endoderm cells under it to form the 

 roof of the enteron. In most amphibians the chorda does not form a broad plate of 

 cells, but is a narrow rod closely united ventrally with the endoderm. which forms 

 the roof of the enteron, and connected laterally with the mesoderm. In these 

 three groups of chordates the chorda plate is widest in ascidians and narrowest 

 in amphibians. In all three it lies in the dorsal lip and is connected laterally 

 with mesoderm (text figs. XXXVI XXXVIII). The later history of the chorda 

 is essentially the same in all three classes. 



The question whether the chorda is of endodermal or of mesodermal origin is. 

 as has been frequently said, one of definition of terms. Castle concludes that it is 

 mesodermal because in Amphioxus and lower vertebrates it " is derived from a 

 common fundament with what is universally regarded as mesoderm" and also 

 because it "comes to occupy a position between the inner and outer layers of the 

 embryo." On the other hand, the histological structure of the chorda cells in 

 Cynthia and Ciona is much more like endoderm than mesoderm, and they are 

 unquestionably derived from cells of the gastric endoderm at the 32-cell stage (fig. 

 117, 103). I believe that special importance should attach to the structure of the 

 cells which form the chorda, and if this be accepted as a guide the chorda, at least 

 among ascidians. should be regarded as endodermal. 



8. Origin of Mesoderm. 



The exact place and manner of origin of the mesoderm of ascidians can be 



recognized with the greatest certainty in the gastrula, cleavage stages and even in 



the unsegmented egg. The crescent, from which most if not all of the mesoderm 



arises, lies just below the equator of the unsegmented egg, and on the posterior 



