31 



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 

 segmentation is complete, and bilateral, and nearly equal, 

 and results in the formation of a spherical blastula with 

 a small segmentation cavity (PI. 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 gastrnla with an archenteron. The hypo- 

 blast cells lining 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 (PI. V., fig. 4, h.p). 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 

 groove (the medullary groove) runs forwards, bounded 

 at the sides by laminae 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 tbe median line form a band 



* Tl)e early stages of C'iona, of which Castle lias given a veiy complete 

 account, ditfer in some [loinls from those of Asisiiliu (Jcsci'ihcd hcic. 



