No. 556] 



ALPHEUS HYATT 



199 



stages that appear are in the main regressive. Nauti- 

 loids and ammonoids, which are characterized by close- 

 coiled shells, build loose-coiled or even uncoiled addi- 

 tions; specialized Ammonites with complex septa, in 

 senescence build simpler septa. Palaeozoic Echini, which 

 are characterized by many columns of plates in an inter- 

 ambulacral area, lose some of these columns in old age 

 growth, all in these features taking on simpler characters 

 comparable to those seen in their own youth, and also 

 comparable to the characters of adults in regressive 

 series in their own groups. 



As an aid in describing stages, Professor Hyatt 5 de- 

 vised a classification of stages in development and decline 

 which is a great convenience in exact description. In 

 this classification the ontogeny is primarily divided into 

 embryonic and postembryonic periods, the latter being 

 for the most part the more important in phylogenetic 

 work. Of embryonic stages the protembryo is repre- 

 sented by the egg and segmentation stages of the same, 

 comparable to the simple and colonial Protozoa as adult 

 forms. The mesembryo is the blastula stage, with a 

 single layer of cells on the periphery of a hollow sphere, 

 comparable to Voir ox and Eudorina, the Mesozoa of 

 Hyatt. The metembryo is the gastrula stage, compara- 

 ble to the simplest of the sponges. The neoembryo is a 

 later stage represented by the early ciliated cephalula 

 stage of a brachiopod and the trochosphere of a mollusc, 

 comparable to the embryo of cha?topod worms and other 

 Coelomata. The typembryo is that stage in development 

 when the features of the great group to which the animal 

 belongs appear. In Mollusca the shell gland and plate- 

 like beginnings of the shell appear at this stage. In 

 brachiopods, two folds of the second segment of the em- 

 bryo turn forward and the corneous shell begins to 

 appear. The phylembryo is the completed embryonic 

 stage and is the first ontogenetic stage that is applicable 



