502 



DEVELOPMENT OF PRIMITIVE BODY FORM 



GASTROCOEL 



NFURAL ECTODERM— Jl.^S:^ih''£ 

 (NEURAL PLATE) 



NOTOCHORD 

 MESODERM 



Fig. 248. Sinking of neural plate and epidermal overgrowth of neural plate in Ainphi- 

 oxus. (Slightly modified from Conklin, '32.) (A) Sagittal section of embryo comparable 

 to that shown in fig. 247F. (B, C, D) Sections through embryo as shown by lines B, 

 C, D, respectively, on (A). Observe that the neural plate begins to sink downward from 

 region of closed blastopore and proceeds forward from this point. 



b. Formation of the Mouth, Anus, and Other Specialized Structures 



of the Metenteron 



At the anterior end of the metenteron, a broad, dorsal outgrowth occurs 

 which continues up on either side of the notochord and becomes divided into 

 right and left dorsal diverticula (fig. 249B, H). The left diverticulum remains 

 small and thick-walled and later fuses with an ectodermal invagination to 

 form the pre-oral pit, described as a sense organ. The right diverticulum, 

 however, increases greatly in size, becomes thin-walled, and gives origin to 

 the so-called head cavity. 



The mouth develops at a time when the larva acquires about 16 to 18 pairs 

 of mesodermal segments or somites (fig. 249D). It appears when the over- 

 lying epidermis about halfway up on the left side of the body fuses with the 

 entoderm, a fusion which occurs just posterior to the forming pre-oral pit 

 (left diverticulum). (See black oval fig. 249D, and fig. 249F.) 



At the time that the mouth forms, the entoderm opposite the first pair of 

 somites pushes ventrally and fuses with the ectoderm. This area of fusion 

 finally perforates and forms the first gill slit. The gill slit, once formed, moves 

 up on the right side of the body (fig. 249E). The entodermal area from 

 which the first and later gill slits make their appearance is known as the 

 branchial rudiment (fig. 249D). 



At the caudal end of the larva, following the degeneration of the neuren- 

 teric canal, a small area of entoderm fuses with the ectoderm and forms the 

 anal opening. The anus is first ventral in position, but later moves up to the 

 left side as the caudal fin develops (fig. 249E, G). 



