424 



TUNIC AT A. 



sac (Figs, l'11 C, 212 A, a and b). The first differentiation in the 

 embryo consists in the appearance of a continuous superficial cell- 

 la yer, in which we recognise the ectoderm of the embryo (Fig. 212 

 A, ec).* Important changes in the brood-sac take place at the same 

 time, its outer lamella, which represents a layer continuous with the 

 epithelium of the atrial-cavity of the parent (Fig. 212, a) soon chang- 

 ing into an extremely thin pavement-epithelium. The inner lamella 

 of the brood-sac (Fig. 212, b) represents the modified epithelium of 

 the oviduct and the follicle. In it we can distinguish a simple cell 

 layer which at first completely encircles the embryo. This is the 

 inner lamella of the brood-sac in the strict sense of the term, and is 



-ec 



Fiii. 212.— Two sections through embryos oi Salpa democratica-mucronata (diagram- 

 matic, after Salensky). .1. younger stage; /.'. sagittal section through an older 



stage, a, outer lamella of the brood-sac; h. inner lamella of the same; ec, ecto- 

 derm [layer ot kalymmocytes, ectoderm not yet formed, Korotneff] ; i. inner cell- 



mass (entoderm, mesoderm?); /. rudiment of elaeoblast; 



mesoderm ( .' 



rudiment of the nerve-centre ; j*. remains ot the follicle = rudiment of the placental 

 tissue ; c, ectodermal thickening, from which the covering of the placenta is derived 

 [this is follicular, i.e., maternal in origin according to Korotneff], 



no doubt derived from the modified oviduct. To the base of the sac 

 formed by the inner lamella is attached a cell -accumulation (p) which 

 probably represents the modified cell-material of the follicle ; this 

 forms the first rudiment of the placenta. [The placenta, including 

 the basal plate, according to Kokotneff (Xo. XX".) is wholly 

 maternal in origin. 



•[According to Korotnkff (No. XVIII.) the embryo becomes covered in the 

 first instance by a layer of kalymmocytes; these Salensky mistook for the 

 ectoderm. The latter layer forms later by a rupture in trie layer of kalym- 

 mocytes covering the embryo and a discharge of blastomeres through the gap 

 into the follicular cavity, where they become arranged as a layer, the future 

 ectoderm between the follicle and the embryo. AIoseneh\ ine-eells are also dis- 

 charged into this cavity, and arrange themselves under the ectoderm. — Ed.] 



