226 



EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



the one just described has the appearance shown in Fig. 124, A 1 . A shallow 

 groove subsequently divides the cumulus into a darker anterior, and a lighter 

 posterior part (Fig. 124, A 2 .)-, and finally a faint depression appears on the right 

 and left, and on the anterior and posterior margins of the cumulus. A 3 . 



In still older specimens, the anterior half of the previous cumulus now appears 

 as a faint, wave-like ridge, in front of the reformed cumulus; the ridge repre- 

 senting the forehead, or procephalic lobes. B 1 ., pr.c. On the anterior half of the 



pr.c I 



c. 



op 



1 1 a 



s 1 ' 



pbl. 



D 



,v t 

 a,p. 

 ...t.p. 



&> 



ap. 



FIG. 125. Limulus embryos, seen as opaque objects, showing the formation of the first six thoracic metameres, 

 and the gradual infolding of the proliferating cells in the posterior cumulus to form a primitive streak, or telopore. 



cumulus, is the beginning of the second thoracic metamere, s 2 ; the posterior 

 germinal spot marks the beginning of the telopore, p.c. The anterior germinal 

 spot lies between the procephalon and the second metamere, a.c. 



In a short time, the posterior half of the primitive cumulus rounds out into 

 a new cumulus with a germinal depression on its posterior side B 2 and B 3 . The 

 remaining thoracic metameres are formed in a similar manner, by successive, 

 wave-like elevations, on the anterior slope of the terminal cumulus, or anal plate. 

 (Fig. 125.) 



The second, third, and fourth thoracic metameres form in regular order, and 

 are of about equal proportions. There is then a distinct pause, followed by the 

 appearance of the fifth and sixth metameres. (Fig. 125, D s .) 



