A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 313 



numerous short, blunt-pointed processes. Each villus consists 

 of an aggregation of round, transparent, vesicles or cells, without 

 nuclei. 



The glassy layer which lies upon the yelk is altogether homo- 

 geneous, but is otherwise remarkable from the first- mentioned 

 structures which are imbedded in it *. Each of these bodies 

 consists of densely aggregated vesicles or cells. We find these 

 structures sometimes solitary, sometimes united into groups of 

 different forms and sizes, in the glassy layer. The latter is, 

 however, nothing else than the primitive rudiment of the 

 mantle, which is already present in the unfecundated ovum of 

 the future Phallusia. The green structures, which persist un- 

 changed during the whole of larval life, change, after metamor- 

 phosis has taken place into the granules, which are contained 

 in abundance in the mantle of the adult animal. This layer 

 has already been recognized by Milne-Edwards (Obs. sur les 

 Ascid. composees des Cotes de la Manche, p. 26. pi. 4. 

 fig. 4) in the ova of Amouroucium proliferum, and has been 

 quite justly interpreted to be the future test (Mantelschicht), 

 as the following passage especially shows, p. 36 : " La couche 

 tegumentaire (L e. the test) est dans le principe la couche gela- 

 tineuse (glassy layer) qui dans Pceuf revet en dehors la masse 

 vitelline." According to this, however, Kolliker's view (An- 

 nales d. Sc. Nat. 1846, t. v. p. 218), that the test in Amourou- 

 cium Nordmanni and Aplidiwn gibbulosum, arises only after the 

 cleavage of the yelk, is confuted ; and I cannot doubt that it 

 is this layer, which has been regarded by Van Beneden (Mem. 

 sur 1'Embryogenie, &c., des Ascidies, in the Mem. de PAcad. 

 Roy. de Bruxelles, t. xx. p. 37) as the albumen of the egg, 

 although Milne-Edwards expressly draws attention to the pos- 

 sibility of falling into this error. 



2. Yelk Cleavage and Embryo. Without doubt the ova, as 

 Cuvier already supposed, and Von Baer has endeavoured to 

 demonstrate with more conclusive arguments, are fecundated in 

 the so-called cloaca, into which the seminal canal and the ovi- 



* The remarkable green or yellowish green colour of the ovarium and of 

 the oviduct, in many Phallusice, arises entirely from these green structures. 

 When the latter are colourless, as in Clavelina for example, the parts in 

 question are without colour. 



