THE HYPOMERE (LATERAL PLATE MESODERM) 231 



tubules of the otherwise normal male frog, and hermaphrodites have 

 been described. These facts simply emphasize the fundamental similar- 

 ity of the frog ovary and testis, both in origin and in early develop- 

 ment. 



The Hypomere (Lateral! Plate Mesoderm) 

 The Coeloni (Splanchnocoel). 



This splanchnocoelic cavity arises as irregular spaces in the lateral 

 plate mesoderm, ventral to the mesomere. These spaces become con- 

 fluent to form a continuous split, the coelom. The cavity therefore 

 develops between an outer somatic and an inner splanchnic layer of 

 mesoderm. The earliest appearance of any coelomic space is in the 

 tail bud stage, and in the anterior heart mesenchymal area. There 

 shortly develop three continuous divisions of the coelom, the muscle 

 coelom (myocoel of the somite), kidney coelom (nephrocoel of the 

 intermediate cell mass), and gut coelom (splanchnocoel of the lateral 

 plate). In addition, there is the heart coelom or pericardial cavity. 



By the time of hatching, the coelomic space (splanchnocoel) on 



(Continued from opposite page.) 

 cavity, pc, filled with embryonic connective tissue. {Top center) The gonad 

 of a 40 mm. larvae, showing the maximum development of the morphologically 

 undifferentiated germ gland. The solid sex cords completely fill the gonad. 

 ( Top right) Section through a young ovary. The secondary genital cavity, sgc, 

 is small. The germ cell nests, gen, are disappearing. The germ cells are in 

 early stages of pseudoreduction. (Center left) The ovary of a 70 mm. larvae, 

 showing the obliteration of the egg nests and secondary genital cavity. A few 

 residual oogonia persist at the periphery of the gland, ro. (Center right) The 

 germ cells have passed from the peritoneum into the sex cords in this gonad 

 and are surrounded by a follicular covering of peritoneal and sex cord elements. 

 (Bottom left) Section through a gonad, showing the first signs of differentiating 

 into a testis. Large irregular primary genital spaces filled with embryonic con- 

 nective tissue are present. At ct are cords of cells passing out to germ cells in 

 the epithelium. (Bottom center) Young testis, showing the secondary genital 

 cavity at sgc and two germ cells enclosed within a cross tubule from the sex 

 cords at ct. Note the formation of other cross cords growing out to the germinal 

 epithelium. (Bottom right) Testis showing the nests of germ cells — anlagen of 

 testis ampullae. Note that the sex cord cells have become organized into vasa 

 efferentia at the hilus and short tubules pass out to enter the ampullae. (From 

 'The Germ Cells of Anurans," by W. W. Swingle. Reprinted from /. MorphoL 

 Physiol, 41, No. 2, March 1926.) 



