314 Avicrican Quarterly Microscopical Journal. 



presence of two layers of cells, instead of a single one, each con- 

 sisting of several rows of cells, the inner layer being formed of 

 cells larger and darker in color than those of the outer. The 

 gastrula also possesses an opening through which refuse matter is 

 excreted and nourishment is received. 



The embryo now assumes a worm-like form, and in its simplest 

 form is allied to the Turbellaria. The main distinction between 

 this and the gastrula stage lies in the number and character of 

 the cellular layers, now four in number. 



From the worm stage branch off the articulates on the one side> 

 the molluscs on the other ; the vertebrates constituting the central 

 stem. 



Following the worm, the embryo becomes a vertebrate, but of 

 a verv simple and imperfectly developed form, for which Hackel 

 has established the sub-type Acrania. 



In the next stage the nervous system and vertebra2 become de- 

 finately formed, and this is known as the monorrhina condition. 



The human being then becomes a fish, and finally reaches the 

 last, or ammion stage. 



Thus we have briefly outlined the; changes through w^hich the 

 human embryo passes in its course of ontogenetic development, as 

 viewed by the German histologist. To bring them forward with 

 their full force would require a translation of the volumes he has 

 written. 



Each of these various stages is, or should be, according to 

 HackeFs view, represented by an adult living animal, which has 

 reached one of these stages and there completed its course of 

 development, having jjreviously passed through each of the lower 

 ones. 



It would be interesting to relate how perfectly observed facts 

 agree with the theory in the minutest details ; also to notice the 

 cases in which the living representatives of the types, previously 

 established upon theoretical grounds, have been discovered. We 

 can only mention a few which will be familiar examples. 



ComiTiencing with the monerula and amoeboid stages, which 

 have already been described, the morula succeeding them finds its 

 representative among certain species of Cystophrys and Labryn- 

 thulece. The blastosphsre is represented by numerous forms, the 

 most familiar, perhaps, being among the Volvocinae. 



The gastrula is familiar in the larval forms of sponges, corals, 



