ASCIDTA. 67 
these parts have been formed in essentially the same 
manner in both cases; so that if similarity in structure 
and development indicate relationship, it is evident that 
the young Ascidian is related to the fish and the frog and 
other Vertebrata, and is to be regarded as one of the 
Chordata (animals which at some time of their life have a 
notochord). 
Turning now to the details of the development,* the 
seomentation is complete, and bilateral, and nearly equal, 
and results in the formation of a spherical blastula with 
a small segmentation cavity (Pl. V., fig. 2). 
The blastula then grows larger and begins to differen- 
tiate. There are slightly smaller cells which divide more 
rapidly at one end of this embryo, the future ectoderm, 
and slightly larger and more granular cells at the other, 
which become chiefly endoderm (hypoblast). Invagination 
of the larger cells then takes place (fig. 3), resulting in the 
formation of a gastrula with an archenteron. The hypo- 
blast cells ning the archenteron become columnar (hy.). 
The curving and more rapid growth at the anterior end of 
the embryo narrow the primitively wide open blastopore, 
and carry it to the posterior end of the future dorsal 
surface (Pl. V., fig. 4, bp.). The directions of the body 
are now clear. The embryo is elongated antero-posteriorly, 
the dorsal surface is flattened and the blastopore indicates 
its posterior end. Around the blastopore certain of the 
ectoderm cells form a medullary plate along which a 
eroove (the medullary groove) runs forwards, bounded 
at the sides by lamine dorsales which meet behind the 
blastopore. Underneath the posterior part of the medullary 
groove certain of the hypoblast cells from the dorsal wall 
of the archenteron in the median line form a band 
*The early stages of Ciona, of which Castle has given a very complete 
account, differ in some points from those of Ascidia described here, 
