108 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



30 from the equator to a point a little below the equator (text fig. XXXIX); no 

 portion of the nervous system comes from the region of the animal pole and none 

 from the posterior pole. Furthermore, the mouth, which is here a new formation 

 and has nothing to do with the blastopore, does not open through the nerve plate 

 but lies between the anterior end of the neural plate and the animal pole. 



We have here differences of a fundamental order, even in the earliest stages of 

 development, between the vertebrate, or rather chordate, and the invertebrate; the 

 early development throws no light upon the way in which the one may have been de- 

 rived from the other. It is of course possible to conceive of a condition in which the 

 nervous system surrounded the entire blastopore as a ring, which in the case of the 

 annelids underwent concrescence from behind forward, thus forming the ventral 

 plate and ganglia, but which in the chordates underwent concrescence from in front 



XXXIX 



end. 



v -9- 



XL 



Figs. XXXIX, XL. Diagramatic representations of the types of germinal localization in ascidians 

 and annelids. Mesodermal substance is shaded by lines, neural substance by fine stipples, and chorda 

 material by coarse stipples. Fig. XXXIX, the ascidian type; egg viewed from right side. The meso- 

 plasm, composed of mesenchyme (m'ch) and muscle substance (msl, is represented in its final position, 

 which it assumes before the first cleavage. The neural plate (n. p.) and chorda (eh.) substances are not 

 distinguishable in the unsegmented egg. but are here shown in the positions in which they appear at the 

 2-cell stage; the chorda and mesenchyme substances should be shown as meeting on the side of the egg, 

 thus forming a chorda-mesenchyme ring around the endoderm. The ectoplasm (ect.) and endoplasm 

 (end.) are localized, as here represented, at the close of the first cleavage. Fig. XL, the annelid type; 

 egg viewed from left side. The substances of the first and second somatoblasts (the former stippled along 

 one border, the latter shaded by lines) are shown in the positions in which these cells are ultimately 

 formed ; in the unsegmented egg the lobe which contains the substances of these cells lies nearer the 

 lower pole. The substances of the cerebral ganglion (c. g.), ventral ganglia (v. g.) and prototroch (proto.) 

 are not distinguishable in the unsegmented egg, but are shown in the regions to which they may be 

 traced by means of the cell lineage. 



backwards. But however probable such a theory may be it finds little support in 

 the early development of ascidians. It is true that a nerve ring has been described 

 as surrounding the blastopore in ascidians, but I have not been able to find evidence 

 of its existence. Furthermore, there is no evidence in the development of ascidians 

 that there is any concrescence of the anterior lip of the blastopore ; on the contrary 

 the anterior lip grows backward over the archenteron as rapidly in the mid-line as 

 at the sides, a view in which practically all writers on ascidian embryology agree. 

 Finally, the lack of an apical plate and cerebral ganglion at the animal pole in the 

 ascidian constitutes a notable difference from the condition found in most inver- 

 tebrates. In his great work on Sa/pa, Brooks (1893) has shown in masterly fashion 



