Report <>t the State Geologist, 



535 



;nv developed in the iinsegmented condition they are retained 



during the blastula and gastrnla stages. With the completion 



^-^v of the primary segmentation and the for- 



\ niation of the segmentation cavity or 



archenteron, follows the division of the 

 embryo into a cephalic and caudal lobe, 

 and the formation of two lateral diver- 

 ticula from the segmentation cavity. This 

 is the primary cephalula stage of the 

 neoembryo, which becomes completed 

 by the development of a third, a median 

 or thoracic lobe, from which four 

 fig. 3oo.- Terebratuiina; a vary bundles of long seta3 are extended 

 J :ZCZtZTlT:tvosterioTly beyond the caudal lobe' 

 ductor muscles, x 20 (Morse). (Cistella). The cephalic and caudal 

 lobes are cilated but the median lobe is not. At the 

 same time a single pair of pigment-dots or eyes devel- 

 oped on the cephalic segment. In a later condition a 

 second pair of eye spots appears, the thoracic or median 

 lobe becomes elongated and its outer layer 

 (mantle) differentiated from the body of 

 the lobe. During this stage the embryo 

 may become attached by its caudal segment 

 or pedicle (Terebratulina). The two divi- 

 sions or lappets of the mantle, which in 

 the last stage embraced the thoracic lobe 

 are now inverted or revolved upon their 

 basis of attachment, until they gradually 

 inclose the cephalic lobe, and their margins, 

 with the bundles of setae, are directed ante- 

 riorly instead of posteriorly. This initiates 

 the typemmyo stage. 



At this time Cistella becomes attached by its caudal segment 

 though the pedicle in Glottidia is not as yet developed. 



The cephalic lobe now rapidly becomes diminished in size. 

 The digestive tube had already been formed by the tubular 

 enfolding of the walls of this lobe. The muscles in Cistella 

 are now in four pairs, two of which extend from the walls of 

 the digestive cavity to the bases of the marginal setaa ; these 

 afterward become the adductor muscles of the adult animal. 



87 



Fig. 301.— Terebratulina; a 

 young shell having the 

 punctae developed, x 20 

 (Morse). 



