ORGAN- FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 22Q 



PHOTO 17. Advanced gastrula, posterior view, superficial focus. The blasto- 

 pore groove is a narrow slit bounded on each side by four large muscle cells, which 

 are derived from the yellow crescent ; the lighter colored cells at the bottom of the 

 groove are mesenchyme and are derived from the middle part of the crescent. The 

 dorsal lip of the blastopore (not clearly shown in the photo) closes the groove dorsally 

 and anteriorly. 



PHOTO 18. Late gastrula, dorsal view, superficial focus. The U-shaped group 

 of mesoderm cells (Photo 17) is seen from the open end of the U ; the ectoderm over- 

 growing the mesoderm is seen as a light area posterior to the yellow cells ; the light 

 wedge-shaped area in the mid-line is the beginning of the neural groove ; the apex of 

 the wedge lies between the limbs of the U and marks the point at which the blastopore 

 closes and also the posterior limit of the neural plate. The ectoderm can be seen as 

 a zone of clear cells with transparent nuclei around the periphery and faint indica- 

 tions of these cells with their clear nuclei can be seen forming seven or eight trans- 

 verse rows of cells across the embryo anterior to the blastopore (the neural plate). 

 The gray endoderm seen through these ectoderm cells gives a dark appearance to all 

 the embryo save the periphery. 



PHOTO 19. Late gastrula, dorsal view, deep focus. This stage is later than the 

 preceding and the embryo is tilted slightly toward the right side, so that the plane of 

 symmetry is a little to the right of the middle of the photo ; the mesoderm cells are 

 no longer transverse to the long axis but are extending in an antero-posterior direc- 

 tion. In front of the mesoderm is a dark area, the endoderm, in which four trans- 

 verse rows of cells may be indistinctly seen ; around the entire periphery is the clear 

 ectoderm. The anterior portion of the embryo is wider and the posterior part narrower 

 than at any previous stage. 



PHOTO 20. Young tadpole, ventral view, superficial focus ; the larva is slightly 

 tilted to the right so that the ventral mid-line lies to the right of the middle of the 

 photo. Three rows of rounded muscle cells (six or seven cells in a row), with clear 

 nuclei, lie on each side of the mid line. In front of the muscle cells on each side is 

 a clear area of mesenchyme. The strand of caudal endoderm cells shows in the mid- 

 line between the muscle rows of each side ; anterior to the muscle and mesenchyme 

 is the gastral endodern. 



PHOTO 21. Young tadpole, ventral view, deep focus. Between the muscle cells 

 is the fusiform notocord which is composed of wedge-shaped cells. Six muscle cells 

 are visible on each side of the chorda and there are several lighter colored mesen- 

 chyme cells at its posterior end. At the anterior ends of the muscle rows is a clear 

 area of mesenchyme cells in which the peribranchial pouches appear. Cells of the 

 gastral endoderm are clearly visible ; around the entire periphery are clear ecto- 

 derm cells. 



PHOTO 22. Young tadpole from left side.. The three rows of muscle cells, each 

 with a clear nucleus are faintly shown in the dark area in the tail, the gastral endo- 

 derm also appears as a dark area in the trunk, but not so dark as the muscle cells. 

 Around the entire periphery is the clear ectoderm ; on the dorsal (convex) side this 

 is especially thick and comprises the neural tube. The hazy areas ventral to the em- 

 bryo are due to aggregated test cells. 



PHOTO 23. Tadpole of about the same stage as the preceding, left-dorsal aspect. 



