PROCESS OF DEVELOPMENT IN BATRACHIAN OVA. 51. "3 



investment of the animal, whilst the inner whitish cells form 

 the proper sensorial lamina. 



In Birds and Mammals these two layers are so intimately 

 connected that no difference can be demonstrated between 

 them, even in the most successful transverse sections. Owing to 

 this, Remak has regarded the two rudimentary structures as 

 one, and has designated the whole lamina as the " sensorial 

 lamina" (central portion), or " corneal lamina" (peripheric 

 portion). He has, however, pointed out that there are theo- 

 retical considerations that are opposed to the view that the 

 corneal and nervous structures originate in one lamina. The 

 fact is therefore of proportionate interest, that in Batrachia, 

 and, as I shall subsequently sh*ow, in Fishes also, the corneal 

 and nervous structures are already distinct at the earliest 

 period of their development. Bearing in mind these circum- 

 stances, I apply the term " corneal lamina" (Hornblatt) to the 

 external layer of brown cells, and " nerve lamina " (Nerven- 

 blatt) to the deeper whitish cell layer. In regard, however, to 

 Birds and Mammals, and generally speaking, where it has 

 become requisite from later investigations, I shall designate the 

 external germ layer (Remak) as the " conjoined corneal and 

 nervous lamina." 



To recapitulate, then, what we have just stated at length, 

 it appears that the corneal and nervous lamina proceeds from 

 an external mantle or investing layer of the spherical ovum, 

 but that the motor and glandular layers originate in the large 

 germ cells collected as a reserve store in the lower half of the 

 ovum. The germ cells have in fact undergone this differentiation 

 in their original position; that is to say, in the lower half of the 

 ovum. But they must be in part also either actively or 

 passively displaced from the inferior towards the superior pole, 

 or, which comes to the same thing, from the caudal extremity 

 of the future larva towards its capitate end. 



As soon as Rusconi's groove is completed into a circle, a 

 fissure runs out from its abdominal half towards the upper 

 pole. It scarcely, however, extends as far as one-fourth of th<> 

 height of the germ cell layer (though its dimensions may vary 

 in different species), and it dilates at the csecal extremity of 

 the ovum. Remak has termed this cleft the anal cavity. 



L L 2 



