704 ADDRESS TO THE DEPARTMENT OF ANATOMY 



place consists in the division or segmentation of the single cell 

 into a number of smaller cells. The cells then arrange them- 

 selves into two groups or layers known to embryologists as the 

 primary germinal layers. These two layers are usually placed 

 one within the other round a central cavity, The inner of the 

 two is called the hypoblast, the outer the epiblast. The ex- 

 istence of these two layers in the embryos of vertebrated animals 

 was made out early in the present century by Pander, and his 

 observations were greatly extended by Von Baer and Remak. 

 But it was supposed that these layers were, confined to ver- 

 tebrated animals. In the year 1849, ar >d at greater length in 

 1859, Huxley demonstrated that the bodies of all the polype 

 tribe or Ccelenterata that is to say of the group to which the 

 common polype, jelly-fish and the sea-anemone belong were 

 composed of two layers of cells, and stated that in his opinion 

 these two layers were homologous with the epiblast and hypo- 

 blast of vertebrate embryos. This very brilliant discovery came 

 before its time. It fell upon barren ground, and for a long time 

 bore no fruit. In the year 1866 a young Russian naturalist 

 named Kowalevsky began to study by special histological 

 methods the development of a number of invertebrated forms 

 of animals, and discovered that at an early stage of develop- 

 ment the bodies of all these animals were divided into eer- 



o 



minal layers like those in vertebrates. Biologists were not 

 long in recognizing the importance of these discoveries, and 

 they formed the basis of two remarkable essays, one by 

 our own countryman, Professor Lankester, and the other 

 by a distinguished German naturalist, Professor Haeckel, of 

 Jena. 



In these essays the attempt was made to shew that the 

 stage in development already spoken of, in which the cells are 

 arranged in the form of two layers enclosing a central cavity has 

 an ancestral meaning, and that it is to be interpreted to signify- 

 that all the Metazoa are descended from an ancestor which had 

 a more or less oval form, with a central digestive cavity pro- 

 vided with a single opening, serving both for the introduction of 

 food and for the ejection of indigestible substances. The body 

 of this ancestor was supposed to have been a double-walled sack 

 formed of an inner layer, the hypoblast, lining the digestive 



