August 9, 1901.] 



SCIENCE. 



203 



same region. This last, however, has no 

 concern for us at present. 



From the angle just mentioned the 

 somatic |^mesothelium now gradually ex- 

 tends upwards and inwards between the 

 cells of the allantoidan trophoblast as a 

 double sheet, dividing the trophoblast of 

 this region into two layers. Finally, the 

 mesothelium growing in from all sides 

 meets and fuses above the embryo, which 

 now lies in a cavity roofed in by an internal 

 layer of ectoderm and an outer of meso- 



but, of course, these are derived in a 

 greatly different way. We see from this 

 that the first splitting in the ectoderm de- 

 scribed above is the separation of amnion 

 from the rest of the ectoderm. In the 

 sauropsida and in some mammals, as is 

 well known, the amnion arises in quite a 

 different manner ; not by the splitting of 

 the ectoderm, but by the upgrowth and 

 overgrowth of folds from all sides of the 

 embryo — each fold consisting of ectoderm 

 and somatic mesothelium — the folds at 



Fig. E. Diagrams of the Origin of the Amnion. 1. Section of an egg of Erinaeeus after Hubrecht, 

 showing the two layers and a cavity in the ectodermic thickening. 2, 3 and 4, diagrams of successive 

 stages of formation of the amnion ,; ectoderm white ; mesoderm dotted ; entoderm black, a, Cavity of 

 amnion ; c, ccelom. 2. Diagram of the stage of Fig. 1, the amniotic cavity formed by splitting of the 

 primitive ectodermic thickening. 3. Appearance of the mesoderm and cnelom, the somatic layer of the 

 mesoderm growing upwards above the amniotic cavity, the roof of which is beginning to split into amnion 

 and serosa. 4. Process of the amnion formation complete, the result closely similar to what is found in 

 reptiles and birds. 



thelium. Between this roof and the tropho- 

 blast there is now a space, lined on either 

 wall by mesothelium, and hence clearly a 

 part of the coelom. The cavity above the 

 embryo is the amniotic cavity, and its 

 ectodermal lining is the amnion. It now 

 shows in every relation exactly the same 

 features as are well known in the chick ; 



last meeting and fusing above the em- 

 bryo, the final result closely simulating 

 that described for the hedgehog. The 

 question now arises, which of these two 

 modes of amnion formation is the primi- 

 tive and which the derived condition? 

 Hubrecht's line of reasoning in settling this 

 question is, in outline, as follows : The am- 



