C. E. Beeeher — Development, of the Brachiopoda. 135 



are called teratologic. The subject of geratology is further 

 divided into the clinologic and nostologic stages. 



The application of this nomenclature of the stages of growth 

 and decline to the Brachiopoda is shown on the following 

 pages. 



Cistella neapolitana Scacchi. 



Figure 1. — Protembryo. Uusegmented ovum. 



Figure 2. — Protembryo. Ovum composed of two spheres. 



Figure 3. — Mesembryo. Blastosphere. 



Figure 4. — Metembryo. Gastrula. (1-4, 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 1, 2, 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 3, has been observed 

 in Cistella, Terebratulina, and Lacazella. The blastula cavity 

 is small. 



The Metembryo, or gastrula stage, figure 4, is developed 

 from the blastosphere in two ways ; (a) by embolic invagina- 

 tion in Cistella and Terebratulina (Kovalevski and Shipley), 

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

 close of this stage, the archenteron in Cistella is trilobed, con- 

 sisting of a central cavity, or mesenteron, connecting on each 

 side with the body cavity. 



The Neoembryo, represented by the trochosphere and seg- 

 mented ciliated cephalula stages, has been more fully observed 

 than any of the preceding. The first advance from the 

 completed gastrula is in the separation of the mesenteron from 

 the body cavity, and the division of the organism into two 

 segments or lobes, the cephalic and caudal, figure 5. Later a 

 third or thoracic segment is developed and carries four bundles 

 of stiff barbed setse, figure 6. The cephalic and caudal lobes 

 are densely ciliated. During the subsequent cephalula period, 

 two eyes, then two others appear in Cistella, and at the same 

 time the dorsal and ventral sides of the thoracic segment 

 become extended over the caudal, and are progressively defined 

 as two lobes, figures 5-9, 24, 25. 



