248 



STUDIES IN EVOLUTION 



To these Jackson n has added the PTiylembryo, taking it from 

 the later stages of the Typembryo to represent the period 

 when the animal can be referred definitely to the class to 

 which it belongs. 



The succeeding stages in the growth of the animal to 

 maturity are termed by Hyatt [and emended by Buckman 

 and Bather] nepionic (young), neanic (adolescent), and ephebic 

 (mature), while old-age characters are called gerontic. The 

 stages are further divided by using the prefixes ana, meta, 

 and para; as anagerontic, metagerontic, and paragerontic. 



The application of this nomenclature of the stages of 

 growth and decline to the Brachiopoda is shown on the fol- 

 lowing pages. 



85 



86 



87 



Cistella neapolitana Scacchi. 



FIGURE 85. Protembryo; unsegmented ovum. 



FIGURE 86. Protembryo ; ovum composed of two spheres. 



FIGURE 87. Mesembryo ; blastosphere. 



FIGURE 88. Metembryo; gastrula. (Figures 85-88, after Shipley.) 



The Protembryo, as in other groups of organisms, includes 

 the ovum and its segmented stages preceding the formation 

 of a blastula cavity. Figures 85 and 86 show protembryonic 

 stages of Cistella. The eggs are spherical, pyriform, or ovoid, 

 and the segmentation proceeds in a regular manner, resulting 

 in a blastosphere composed of equal parts. 



The Mesembryo, or blastosphere (figure 87), has been ob- 

 served in Cistella, Terebratulina, and Lacazella. The blas- 

 tula cavity is small. 



The Metembryo, or gastrula stage (figure 88), is developed 

 from the blastosphere in two ways : (a) by embolic in vagina - 

 tion in Cistella and Terebratulina (Kovalevski and Shipley), 

 and (5) by delamination in Lacazella (Kovalevski). At the 

 close of this stage the archenteron in Cistella is trilobed, 



