CHAPTER XXXVIII. 



DEVELOPMENT OF THE SIMPLE TISSUES. 

 BY S. STKICKER. 



THE status nascens of a vertebrate animal is occasioned by, 

 and coincident with, the impregnation of the female germ. 

 The fecundated germ is a unicellular organism, which becomes 

 by fission multicellular. When the process of division or 

 fission has reached a certain point, the young cells form layers 

 or laminae, and from these the different tissues are developed, 

 the various organs of the body being again formed from the 

 union of these tissues; coincidently with the appearance of 

 the laminae, the differentiation of the tissues begins to take 

 place, and it thus becomes intelligible why the account of 

 the embryonal laminae is worked at with such care by 

 histologists. 



Embryologists also understand by the term "embryonic 

 laminae " the membranes by which the embryo subsequently 

 becomes invested. These investments, however, do not stand 

 in intimate relation to histogenesis ; they are really transitory 

 organs, which, like all other organs, develop from the pri- 

 mary laminae. 



A knowledge of the embryonic investments is therefore a 

 part of the history of development of the several organs 

 and the former cannot be discussed without sketching out 

 the principal features of the latter. 



From these preliminary observations the line to be pursued 

 in the following account is self-evident. The embryonic cell 



