ENLARGED PERICARDIUM IN LEBISTES EMBRYO 



191 



occupied by the yolk. It collapses prior to ovulation and subséquent birth. 

 (Kunz, 1963). 



Grading and désignation of stages: the embryos were graded into 8 stages 

 according to visible steps in development and régression of the pericardial sac. 



JThe embryonic âge given is calculated as described under 2. Material and Methods. 



lit is followed by a) the description of certain characteristics found in the majority 

 of a batch of living embryos of that stage, which are easily recognized under the 

 dissecting microscope, and by b) some diagnostic histological features of the 

 head région. 



Stage 1 (4th embryonic day) 



a) The embryo is marked out as a linear thickening. Brain, with optic 

 primordia, makes its appearance. Up to 3 somites présent. 



b) Brain and optic primordia are solid, but thin crevasses indicate the area 

 where the cells will separate to form a lumen. The auditory placodes appear as 

 epidermal thickenings (Fig. 3). 



Paired solid primordia of the pericardium, lying anteriorly to the auditory 

 placodes, may be seen (Fig. 3). The head lies embedded in the yolk. The 

 nervous ectoderm, over the head of the embryo, becomes thickened and passes 

 insensibly into the cells of the brain primordium. The thickened embryonic 

 ectoderm tapers into the thin ectoderm of the extra-embryonic area (Fig. 2e). 



Stage 2 (4th embryonic day) 



a) Surface view as above. Not less than 3 somites présent. 



b) The clefts within the primordia of the eyes are more pronounced. 



The primordia of the pericardium acquire lumina. At the area between the 

 thickened embryonic and thin extra-embryonic nervous ectoderm, latéral folds 

 appear. Each fold grows in a medio-ventral direction and extends longitudinally 

 from the tip of the brain to the end of the eye région. This infolding marks the 

 beginning of the formation of the ectodermal headfold. The epidermic stratum 

 is not involved in this process (Fig. 2f ). 



Stage 3 (5th embryonic day) 



a) Surface view as above, but 4-8 somites. 



b) A distinct lumen in the eye primordium is 

 :onspicuous; the future ventricles of the brain are 

 still only indicated as a vertical crevasse. The 

 auditory placode has invaginated as a solid mass. 



The pericardium extends anteriorly and its 

 lumina enlarge (Fig. 3). The ectodermal head- 

 Tolds meet and fuse in the mid-ventral line. The 



ap pcp pc ey 



— .1 kn ( 



Fig. 3 



Surface view of embryo at stage 1 

 showing position of pericardial pri- 

 mordia (extension of pericardium 

 up to stage 3 indicated as dotted 

 line) 



